General and Controllable Synthesis Strategy of Metal Oxide/TiO2 Hierarchical Heterostructures with Improved Lithium-Ion Battery Performance Academic research paper on "Nano-technology"

SUBJECT AREAS:

CHEMISTRY

MATERIALS CHEMISTRY

NANOSCIENCE AND TECHNOLOGY

ELECTRONIC DEVICES

Received 2 July 2012

Accepted 12 September 2012

Published 3 October 2012

Correspondence and requests for materials should be addressed to X.Z. (xbzhang@ciac.jl.

General and Controllable Synthesis Strategy of Metal Oxide/TiO2 Hierarchical Heterostructures with Improved Lithium-Ion Battery Performance

Hengguo Wang1, Delong Ma1,2, Xiaolei Huang1,3, Yun Huang1,2 & Xinbo Zhang1

1State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 130022 Changchun, China, 2Key Laboratory of Automobile Materials, Ministry of Education, and School of Materials Scienceand Engineering, Jilin University, Changchun 130012, China, 3Graduate University of Chinese Academy of Sciences, Beijing 100049, P. R. China.

We demonstrate a simple, efficient, yet versatile strategy for the synthesis of novel hierarchical heterostructures composed of TiO2 nanofiber stem and various metal oxides (MOs) secondary nanostructures, including Co3O4, Fe2O3, Fe3O4, and CuO, by advantageously combining the versatility of the electrospinning technique and hydrothermal growth method, for which the controllable formation process and possible formation mechanism are also investigated. Moreover, as a proof-of-concept demonstration of the functional properties of these hierarchical heterostructures, the Co3O4/TiO2 hierarchical heterostructures are investigated as the lithium-ion batteries (LIBs) anode materials for the first time, which not only delivers a high reversible capacity of 632.5 mAh g-1 and 95.3% capacity retention over 480 cycles, but also shows excellent rate capability with respect to the pristine TiO2 nanofibers. The synergetic effect between Co3O4 and TiO2 as well as the unique feature of hierarchical heterostructures are probably responsible for the enhanced electrochemical performance.

Growth of functional semiconductor oxides with controllable hierarchical heterostructures has attracted tremendous attention because diverse properties can be integrated by tailoring the morphology, composition, and assembling organization of the primary nanobuilding blocks. Specifically, novel hierarchical heterostructures, in which the major 1D cores and branches consist of different materials, are more attactive in many nanoscale photonic and electron-optical device applications. Especially, to serve as materials for lithiumion batteries (LIBs), the hierarchical heterostructures not only integrate the diverse properties from the different building blocks, but also exhibit intriguing properties by taking advantage of both the nanometer-size effects and the high stability of the secondary-structure assemblies1-6. For instance, epitaxial growth of branched a-Fe2O3/ SnO2 nano heterostructures could improve LIB performance due to the synergetic effect exerted by SnO2 and a-Fe2O3 as well as the unique branched structure5. Furthermore, a novel high performance cathode material for LIBs has been developed by coating V2O5 on SnO2 nanowires, utilizing the better conductivity of SnO2 nanowires and the short diffusion distance of the thin V2O5 layer6. Notwithstanding these advances, it is still of great challenge to develop a general approach with the characteristics of low cost, low growth temperature, environmental friendliness, process simplicity, and high yield to synthesize a control of size, dimension, and composition of the building blocks of hierarchical heterostructures in an expected manner.

TiO2, as an important functional material, has numerous applications as photocatalysis, electronic devices, conversion devices as well as promising electrode material due to its attractive advantages of high abundance, low cost, better safety, and environmental friendliness7-13. Furthermore, the special structural characteristics and surface activity, together with the very low volume change during Li1 insertion/desertion (3-4%), intrinsically endows TiO2 good structural stability and long cycle life14-17. Nevertheless, the main weaknesses of TiO2 as anode materials for LIBs lie in the low theoretical capacity (168 mAh g1, even lower than that of commercial graphite

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~372 mAh g21) and the poor rate capability of TiO2 electrodes. Recently, different strategies, such as constructing one dimensional (1D) TiO2 nanostructures (nanorod, nanofiber, nanotube, and core-shell nanowire)18-21 and coating or mixing with more conductive materials, such as carbonaceous materials22-24, noble metals (Au, Ag etc)25,26, and RuO227, and/or doping with foreign atoms20,28 have been developed to overcome the ionic and electronic transport limitations and improve the rate capability of TiO2-based anodes. However the strategies mentioned above still suffer from disadvantages such as high cost, complicated production procedure, and to add insult to injury decrease the capacity of TiO2, because these coating materials are not very active materials for reversible lithium storage. Therefore, it is urgently important to develop new strategies that can coat the active materials with high capacity and conductivity on the surface of TiO2 nanostructures. On the other hand, as a new class of promising anode materials for LIBs, transition-metal oxides (MOs) have shown desirable property of higher theoretical capacities (700-1000 mAh g21, which is much higher than that of commercial graphite and TiO2)29-34. However, an intrinsically drastic volume change during lithiation/delithiation process gives rise to pulverization that may break the electrical contact pathways in the MOs electrodes, thus leading to a rapid decay in capacity and limiting the practical use. Therefore, developing new synthetic strategies to fabricate high-performance MOs electrode materials with both large reversible capacity and long cycle life are highly desirable but still very challenging.

Stimulated by the above concerns, we envision building up an integrated smart architecture, in which structural features and elec-troactivities of each component are fully manifested, the interface/ chemical distributions are homogeneous at a nanoscale and a fast ion and electron transfer is guaranteed. Herein, we propose and realize a facile, effective, and general strategy for preparing MOs/TiO2 hierarchical heterostructures by advantageously combining the electro-spinning technique with the hydrothermal method, which holds many favorable merits including: (1) both the electrospinning and hydrothermal method have been proven to be a comparatively versatile, low cost, applicable and environmentally friendly technique. (2) This strategy can be extended to the fabrication of various

hierarchical MOs, such as Co3O4, Fe3O4, Fe2O3, CuO, on TiO2 nano-fibers to form MOs/TiO2 hierarchical heterostructures. (3) The syn-ergistic effects of nanobuilding blocks as well as the unique hierarchical heterostructures may contribute to the improved LIBs performance. With this design, not only all the desired functions of each constituent are effectively utilized, but also a strong synergistic effect can be realized. As MOs feature both high capacity and higher electronic conductivity (compared to TiO2), hierarchical hetero-structures composed of TiO2 nanofiber and various metal oxides (MOs) could simultaneously increase the rate capability and specific capacity of TiO2 and improve the cycling performance of MOs by the synergistic effect. As a proof-of-concept demonstration of the application of such hierarchical heterostructures, the Co3O4/TiO2 hierarchical heterostructures are tested as LIBs anode for the first time, which show high reversible capacity, improved cycling stability, and excellent rate capability with respect to the pristine TiO2.

Results

Fabrication of MOs/TiO2 hierarchical heterostructures. Four typical MOs, namely, Co3O4, Fe2O3, Fe3O4, and CuO, are adopted as secondary nanostructures grown on primary TiO2 nanofiber to verify the feasibility and versatility of our protocol in achieving MO/ TiO2 hierarchical heterostructures. We firstly test this concept with Co3O4 nanosheets grown on primary TiO2 nanofiber to form the Co3O4/TiO2 hierarchical heterostructures. The morphology of the Co3O4/TiO2 hierarchical heterostructures is investigated by scanning electron microscopy (SEM). As shown in Figure 1a, the obtained pristine TiO2 non-woven nanofibers with diameters ranging from 200 to 500 nm have a relatively smooth surface without secondary nanostructures. After applying the controllable solution growth of Co3O4 nanosheets, the hierarchical heterostructures are formed (Figure 1b). Close observation on the nanofiber (Figure 1c) reveals that the high densities of secondary Co3O4 nanosheets grow on the primary TiO2 nanofibers. Interestingly, compared with the hierarchical heterostructures before annealing treatment (Fig. S1a and b), it is obvious that many pores caused by the annealing treatment are found on the surface of the secondary Co3O4 nano-sheets. These pores may exhibit large surface area, which is very

Figure 1 | Morphology characterization. (a) SEM image of pristine TiO2 nanofibers. (b) Low- and (c) high-magnification SEM images of Co3O4/TiO2 hierarchical heterostructures. (d) Typical TEM image of the single Co3O4/TiO2 hierarchical nanofiber. (e) HRTEM image of the heterojunction region. (f) HRTEM image of the Co3O4 nanosheet surface.

precious for LIBs materials. Further information about the Co3O4/ TiO2 hierarchical heterostructures is obtained from transmission electron microscopy (TEM). The TEM image in Figure 1d shows an individual Co3O4/TiO2 hierarchical nanofiber. It can be clearly seen that the secondary Co3O4 nanosheets with many pores grow on the surface of TiO2 nanofibers, coinciding with the results from the SEM observations. The HRTEM image (Figure 1e) taken from the heterojunction region displays two set of lattice fringe spacings of 0.35 and 0.47 nm, corresponding to the (101) plane of the anatase crystal structure of TiO2 and the (111) plane of the cubic Co3O4, respectively, which clearly shows the simultaneous presence of TiO2 and Co3O4 crystal lattices in the region of the junction. Figure 1f shows the further magnified HRTEM image of the nanosheet, the measured lattice fringe spacings of 0.25 nm corresponding to the (311) plane of the cubic Co3O4 can be clearly observed.

To investigate the crystal phase of the samples, the X-ray diffraction (XRD) analysis is performed (Figure 2a). The XRD patterns reveal that all the diffraction peaks could be indexed to anatase TiO2 (JCPDS No. 21-1272) and face-centered cubic phase of Co3O4 (space group: Fd3m (227), JCPDS No. 43-1003). These results confirm that the synthesis strategy adopted successfully achieves Co3O4/TiO2 hierarchical heterostructures integrating the Co3O4 nanosheets and TiO2 nanofibers. Subsequently, the purity, composition and fine structure of the samples are further investigated. In XPS spectra, Fig. S2 shows the characteristic peaks of C 1s, N 1s, O 1s, and Co 2p, respectively, indicating the existence of carbon, nitrogen, oxygen, and cobalt elements in the Co3O4/TiO2 hierarchical hetero-structures. The C 1s and N 1s peaks may be assigned to the pyrolysis of the urea. Obviously, the Ti signal is absent from this XPS spectra. As XPS is a highly surface-specific technique with a typical analysis depth of ~10 nm (lower than the thickness of the secondary Co3O4 shell), it is reasonable that TiO2 is not detected35. This thus provides indirect experimental proof of the uniformity of the secondary Co3O4 nanostructures coating the TiO2 nanofibers. In Raman spectra (Figure 2b), four well-resolved peaks are observed around 143.6 (Eg), 399.1 (B1g), 519.3 (A1g), and 639.7 (Eg) cm21, corresponding to the characteristic Raman modes of anatase TiO2. After the growth of Co3O4 nanosheets, in addition to the peaks from anatase TiO2, the new peaks at 193.6 (B1g), 476.9 (Eg), 517.1 (F2g), 615.1 (F2g), 684.3 (A1g) cm21, corresponding to the characteristic Raman modes of Co3O4, are observed and demonstrate the successful introduction of Co3O4. These results demonstrate the existence of both anatase TiO2 and Co3O4 in the heterostructures.

The synthesis strategy can be readily extended to the growth of other MOs (e.g. Fe2O3, Fe3O4, and CuO) as secondary nano-structures on the primary TiO2 nanofibers, forming MOs/TiO2

hierarchical heterostructures. Similarly, the Fe2O3/TiO2 hierarchical heterostructure can also be easily synthesized through our general strategy. The low- and high-magnification SEM images (Figure 3a and b) show the secondary Fe2O3 nanorods grown on the primary TiO2 nanofibers to form Fe2O3/TiO2 hierarchical heterostructures with diameters of about 290 nm. The structure is further investigated by TEM in more detail. Figure 3c shows that Fe2O3 nanorods are uniformly attached on the primary TiO2 nanofibers surface, and the selected circular area is enlarged (Figure 3c, inset). The diameter and length of Fe2O3 nanorods are estimated to be about 22 and 45 nm, respectively. The HRTEM image (Figure 3d) further confirms the single-crystalline structure of the a-Fe2O3 nanorods with a lattice fringe spacing of 0.37 nm from the (311) plane.

Apart from Co3O4 and Fe2O3, Fe3O4 and CuO nanoparticles grown on the primary TiO2 nanofibers are employed to further demonstrate the efficiency and generality of our strategy. The resulting composites exhibit the hierarchical nanostructure with secondary nanoparticles grown on the major nanofibers (Figure 4). In the case of Fe3O4/TiO2 hierarchical heterostructures, the secondary Fe3O4 nanoparticles with diameters of about 40 nm are grown on the TiO2 nanofiber substrates. In the case of CuO/TiO2 hierarchical heterostructures, the secondary CuO nanoparticles with diameters of about 600 nm are grown on the TiO2 nanofiber substrates. Furthermore, the crystal structure information and the chemical components of the heterostructure are characterized by XRD (Figure 5). Their XRD peaks can be well indexed as rhombohedral hematite a-Fe2O3 (JCPDS No. 33-0664), cubic magnetite Fe3O4 (JCPDS No. 190629) and tenorite CuO (JCPDS No. 48-1548), respectively. Strong and sharp diffraction peaks of the patterns suggest that the as-synthesized hierarchical heterostructures are highly crystallized.

Formation mechanism of Co3O4/TiO2 hierarchical heterostructures. Based on the above analysis, combining the versatility of the electrospinning technique and hydrothermal growth enables the fabrication of MO/TiO2 hierarchical heterostructures. Evidently, no surface pretreatments are needed to introduce new surface functional groups or additional covalent and/or noncovalent interconnectivity in our experiments. Subsequently, we take Co3O4/TiO2 hierarchical heterostructures as an example and investigate the formation mechanism of hierarchical heterostructures, as illustrated in Figure 6.

To get more insight into the actual evolution process of the Co3O4/ TiO2 hierarchical heterostructures, a series of time-dependent experiments is conducted, and the intermediate solids at the different reaction stages (nucleation, growth, etc.) are examined. Figure 6a-f show representative SEM images for the samples collected stepwise

Figure 2 | Phase analysis. (a) XRD patterns of pristine TiO2 nanofibers and Co3O4/TiO2 hierarchical heterostructures. (b) Raman spectra of pristine TiO2 nanofibers, Co3O4 nanostructures, and Co3O4/TiO2 hierarchical heterostructures.

Figure 3 | Morphology characterization. (a) Low- and (b) high-magnification SEM images of Fe2O3/TiO2 hierarchical heterostructures. (c) Low- and high-magnification (inset) TEM images of Fe2O3/TiO2 hierarchical heterostructures. (d) HRTEM image and fast-Fourier transformation (FFT) of the Fe2O3 nanorod surface.

after 1,1.5,2,4,6, and 9 h of reaction. These sequential images reveal a morphological and structural transformation from pristine TiO2 nanofibers to Co3O4/TiO2 hierarchical nanofibers. Evidently, in the initial stage (1 h) of the hydrothermal reaction, there is no visible change to the pristine TiO2 nanofibers (Figure 6a). However, after 1.5 h growth, tiny nanowires nucleated on the surfaces of the TiO2 nanofibers are observed (Figure 6b). When the reaction time is extended to 2 h, although a few nanosheets have already formed, most TiO2 nanofibers appeared to be surrounded by tiny nanowires (Figure 6c). When the reaction time is further extended to 4 h, the continuous growth results in the disappearance of the tiny nano-wires. As a result, the secondary nanosheets grow on the primary

TiO2 nanofibers (Figure 6d). However, by further increasing the reaction time, the overall morphology scarcely shows any change (Figure 6e and f).

Based on the time-dependent experiments, the possible mechanism is proposed to explain the formation of these hierarchical heterostructures (Figure 6g). Firstly, the electrospun TiO2 nanofibers provide negative charges on the surface, which is demonstrated by its negative zeta potential ( — 29.4 mV). It is well known that when two components with opposite charges are carefully introduced together, the mutual electrostatic interactions drive Co21 to attach on the surfaces of the TiO2 nanofibers. Then, the reaction of Co21 and urea takes place and the formed cobalt-hydroxide-carbonate nuclei attach

Figure 4 | Morphology characterization. (a) Low-magnification, (b) high-magnification SEM images and (c) TEM image of Fe3O4/TiO2 hierarchical heterostructures. (d) Low-magnification, (e) high-magnification SEM images and (f) TEM image of CuO/TiO2 hierarchical heterostructures.

Figure 5 | Phase analysis. XRD patterns of (a) Fe2O3/TiO2, (b) Fe3O4/TiO2, and (c) CuO/TiO2 hierarchical heterostructures.

to the surface of pristine TiO2 nanofibers. In the reaction, TiO2 nanofibers are used as the substrate, which can guide the secondary nanoparticles self-assembling growth in aqueous solution without surfactant and stabilizers. In this key step, the uniform small nuclei are very important because they could determine the growth kinetics of the nanosheets on the surface of the TiO2 nanofibers36. Then the oriented attachment can guide the nanoparticles growth37. With the continuously reaction proceeding, the precursors are consumed for the oriented growth to form large-scale nanosheets surrounding the TiO2 nanofibers. With our starting chemical precursors, the formation of the cobalt-hydroxide-carbonate phase can be demonstrated by the XRD patterns (Fig. S3), which agrees well with previous reports38,39. Finally, the cobalt-hydroxide-carbonate transforms into Co3O4 induced by the thermal annealing. Upon annealing for 4 h in air, cobalt-hydroxide-carbonate decomposes gradually and black Co3O4 nanosheets are obtained accordingly. Obviously, large quantities of pores are widely distributed on the surface of Co3O4 nanosheets, the yield of which would mainly rely on the pyrolysis of hydroxide-carbonate and the release of gases (CO2, H2O) during the calcination process40.

Electrochemical performance of Co3O4/TiO2 hierarchical heterostructures. To demonstrate the effectiveness of the TiO2-based hierarchical nanofibers in improving the lithium storage performance, we take Co3O4/TiO2 hierarchical heterostructures as a representative example and investigate the electrochemical properties of them as anode materials for LIBs, since Co3O4 has high theoretical capacity (~890 mA h g21), and is widely considered to be an attractive anode material for next-generation LIBs.

Figure 7a shows the representative cyclic voltammetry (CV) curves ofthe Co3O4/TiO2 hierarchical heterostructures in the voltage range of 0.01-3 V. Evidently, a pronounced reduction/oxidation peak appeared at 1.7/2.1 V is as the same as that reported for anatase TiO2, characteristic for the lithium storage mechanism between tetragonal anatase and orthorhombic LixTiO2 (TiO2 + xLi1 + xe2 < LixTiO2)2,41,42. In addition, the reduction peak at ~0.98 V and ~0.66 V during the cathodic process in the first cycle can be ascribed to the initial reduction ofCo3O4 to Co, the electrochemical formation of amorphous Li2O, and the formation of a partially irreversible solid electrolyte interphase (SEI) layer32-34. It is noted that reversible formation could be electrochemically driven by the metal

Figure 6 | Formation mechanism of Co3O4/TiO2 hierarchical heterostructures. SEM images of Co3O4/TiO2 hierarchical heterostructures (before annealing) at various reaction stages by setting the reaction time 1 h (a), 1.5 h (b), 2 h (c), 4 h (d), 6 h (e), and 9 h (f). And schematic of the formation process of Co3O4/TiO2 hierarchical heterostructures (g).

nanoparticles formed in situ. Hence, the presence of Co nanoparti-cles at the interface between TiO2 and Co3O4 may improve the reversibility reaction of the anatase TiO2 and further result in a reversible capacity5. In general, during the anodic process, the oxidation of metallic Co to Co3O4 and the decomposition of Li2O could result in the appearance of the oxidation peak at ~2.1 V32-34. Interestingly, in present case, only one oxidation peak at about 2.1 V is observed, it is obvious that these two oxidation peaks coincide with each other. Note that, the second and onward CV curves remain steady, indicating the excellent cycle stability of the hierarchical heterostructures. Subsequently, the lithium storage performance of the Co3O4/TiO2 hierarchical heterostructures is investigated. Figure 7b shows the discharge and charge curves at a current density of 200 mA g21. Consistent with the above CV analysis, the similar current peaks can be identified. These voltage profiles are characteristic of both Co3O4-based and TiO2-based materials. It is observed that the initial discharge and charge capacities of the Co3O4/TiO2 hierarchical heterostructures electrode are 632.5 and 499.7 mAh g21, respectively, corresponding to an irreversible capacity loss of 21.0%. The capacity of the Co3O4/TiO2 hierarchical heterostructures electrode does not decay, and it can deliver a reversible capacity of 602.8 mAh g21 even after 480 cycles (Figure 7b and c). Importantly, the Co3O4/TiO2 hierarchical heterostructures electrode still exhibits an excellent cyclic performance at a higher current density of 500 mA g21, and the capacity still reaches 302.9 mA g21 after 250 discharge and charge cycles (Fig. S4). In contrast, the pristine TiO2 nanofibers electrode only exhibits a very lower capacity (initial discharge and charge capacities of 275.8 and 159.8 mAh g21), and the single Co3O4 nanostructures electrode loses the capacity severely

(the discharge capacity of 381.4 mAh g21 after only 90 charge-discharge cycles) (Fig. S5). Evidently, the composite hierarchical hetero-structures inherit the advantages of high capacity from Co3O4 component and the excellent stability of TiO2 component. To better understand the advantage of the hierarchical heterostructures in lithium storage, the rate performance ofthe Co3O4/TiO2 hierarchical heterostructures is investigated (Figure 7d). Obviously, compared with the pristine TiO nanofibers, the specific capacity of the hierarchical heterostructures is substantially increased at all investigated charge-discharge rates from 100 to 2000 mA g21. For example, the Co3O4/TiO2 hierarchical heterostructures exhibit much superior rate performance with 475.8 and 449.5 mAh g21, which is about three times larger than that of pristine TiO2 electrode (143.2 and 115.3 mAh g21), at current densities of 400 and 1000 mA g21, respectively. At the high current rate (2000 mA g21), the hierarchical heterostructures exhibit a downward trend, which can be ascribed to the partial collapse of the electrode materials during the chargedischarge at a high current density. Further investigations are ongoing to improve the capacity retention at high discharge rates. However, the difference of rate performance between the two materials is quite more remarkable even at high current density. Importantly, the high capacity of the hierarchical composite electrode can be recovered to the initial values even after 70 discharge and charge cycles, implying their good reversibility.

Discussion

The above obtained improved electrochemical properties of the Co3O4/TiO2 hierarchical heterostructures are not simply a result of the mix of the two components. Instead, it more likely originates

Figure 7 | Electrochemical properties of Co3O4/TiO2 hierarchical heterostructures. (a) CV of Co3O4/TiO2 hierarchical heterostructures at a scan rate of 0.1 mV s21 in the range of 3-0.01 V. (b) Discharge and charge curves of Co3O4/TiO2 hierarchical heterostructures at a current density of 200 mA g21 in the range of 3-0.01 V. (c) Cycling performance and coulombic efficiency of pristine TiO2 nanofibers and Co3O4/TiO2 hierarchical heterostructures at a current density of 200 mA g21. (d) Rate performance of pristine TiO2 nanofibers and Co3O4/TiO2 hierarchical heterostructures.

Figure 8 | Electrochemical impedance spectra of Co3O4/TiO2 hierarchical heterostructures. Nyquist plots before cycling for the pristine TiO2 nanofibers, single Co3O4 nanostructures and Co3O4/TiO2 hierarchical heterostructures by applying an AC voltage of 5 mV amplitude at 0.1 Hz to 700 kHz.

from the successful integration of the respective advantages from both TiO2 and Co3O4 individual components into the unique hierarchical heterostructures, which is elaborated as follows. On one hand, the improved capacity of hierarchical heterostructures compared with the pristine TiO2 nanofibers can be easily understood by the addition of a higher capacity Co3O4 component and the synergistic effect of Co3O4 nanosheets and TiO2 stems mentioned above. It is well-known that one advantage of transition-metal oxides in lithium storage is their high theoretical specific capacity. Hence, a large content of Co3O4 could give rise to a high specific capacity. On the other hand, the synergistic effect of the successful integration of the Co3O4 nanosheets and TiO2 nanofibers contributes more to the improved cycling stability and excellent rate capability of the Co3O4/ TiO2 hierarchical heterostructures. Herein, the reason can be the following: the Co3O4 nanosheets with many spaces each other have an increased portion of exposed surfaces and ensure a high utilization of electrode materials, thus a high capacity. Besides, the ultralong hierarchical heterostructures can suppress the self-aggregation, buffer the volume changes of Co3O4 nanosheets, and keep the effective contact areas of active materials and electrolyte large and sufficient during the cycling, thereby resulting in better cyclic stability5,43. Specifically, in comparison with the pristine TiO2 nanofibers, the highly dense Co3O4 nanosheets grow on the TiO2 nanofibers substrate orderly. This structure can provide more electrochemically active sites and sufficient order to enable Li1 access to the hetero-structures facilely. In addition to the improved cycling stability, the Co3O4/TiO2 hierarchical heterostructures also exhibit excellent rate capability with respect to the pristine TiO2 nanofibers. Therefore, the difference in the conductivity between them should be taken into account. To probe the kinetic properties of the hierarchical hetero-structures, we take the electrochemical impedance spectroscopy (EIS) measurements. All three plots show partially overlapping semicircles at high-to-medium frequencies, and a line was observed at low frequencies in Figure 8. Apparently, the charge transfer resistances Rct for the three samples is in the ascending order of TiO2 (730.0 V) > Co3O4/TiO2 (170.9 V) > Co3O4 (137.2 V), whilst the contact resistances (see the intercept of the real axis in Figure 8) between electrolyte and electrode materials are similar, which indicates that the secondary Co3O4 nanosheets coating could enable much easier charge transfer at the electrode/electrolyte interface (compared with TiO2 nanofibers) and boost the electronic conductivity. Thus, the improved electronic conductivity is considered to be a key factor in improving the rate capability and cyclic life of the Co3O4/TiO2 hierarchical heterostructures.

In summary, a simple, general yet effective strategy is developed based on the advantageously combination of the versatility of the electrospinning technique and hydrothermal growth to achieve various MOs, including Co3O4, Fe2O3, Fe3O4, and CuO, grown on pristine TiO2 nanofibers to form MO/TiO2 hierarchical heterostruc-tures. As potential anode materials for LIBs, the as-formed Co3O4/ TiO2 hierarchical heterostructures exhibit high reversible capacity, improved cycling stability and rate capability with respect to the pristine TiO2 nanofibers. The improved electrochemical performance is ascribed to the synergistic effect of the advantageous integration of Co3O4 and TiO2 hierarchical nanostructure. The proposed synthesis strategy would open up new opportunities in the development of high performance next-generation LIBs used for alternative energy and electric transportation. Such hierarchical heterostruc-tures might also be used in broad fields including photoelectrochem-ical water splitting and gas sensors.

Methods

Sample synthesis. The MO/TiO2 hierarchical nanostructures were prepared by the electrospinning and hydrothermal method44. Typically, 0.2429 g Co(NO3)2'6H2O and 0.25 g urea were dissolved in 25 mL deionized water under magnetic stirring. Then the resulting solution was transferred into a 50 mL Teflon-lined autoclave with electrospun TiO2 nanofibers (20 mg). The autoclave was sealed and heated at 120°C for 6 h, and cooled to room temperature. The film was collected out, washed with ethanol and deionized water, respectively, and then dried under vacuum at 50°C for 12 h. Then the as-collected films were calcined at 400°C for 4 h. For comparison, the single Co3O4 nanostructures were also prepared under the same condition instead of the addition of TiO2 nanofibers. When Co3O4 precursor solution was replaced by Fe2O3 precursor solution (0.243 g FeCl3 and 30 mL deionized water), Fe3O4 precursor solution (0.25 g FeCl3'6H2O, 0.25 g polyethylene glycol, 0.9 g sodium acetate and 25 mL ethylene glycol), and CuO precursor solution (0.2557 g CuCl2'2H2O, 0.3 g urea and 30 mL deionized water), and the autoclave was sealed and heated at 90°C for 6 h (Fe2O3), at 200°C for 16 h (Fe3O4), or at 180°C for 12 h (CuO), then the as-collected films were calcined at 500°C for 2 h except Fe3O4/TiO2 hierarchical nanofibers.

Sample characterization. The morphology and crystalline structure of as-obtained samples were characterized with scanning electron microscopy (SEM Hitachi S-4800) and transmission electron microscope (TEM) recorded on a Tecnai G2 operating at 200 kV. X-ray diffraction (XRD) patterns were recorded on a Rigaku-Dmax 2500 diffractometer with Cu Ka radiation. X-ray photoelectron spectroscopy (XPS) analysis was carried on an ESCALAB MK X-ray photoelectron spectrometer. Zeta potential was measured by dynamic light scattering (Malvern Nano-ZS). Raman measurements were performed at room temperature using an inVia Reflex Raman spectrometer with Raman shift from 100 to 1000 cm21.

Electrochemical evaluation. The electrodes were prepared by mixing active materials (70 wt%), acetylene black (20 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) in N-methyl-2-pyrrolidone (NMP). After the above slurries were uniformly spread onto a copper foil, the electrodes were dried at 80°C in vacuum for 6 h. Then the electrodes were pressed and cut into disks before transferring into an Argon-filled glove box. Coin cells (CR2025) were laboratory-assembled using lithium metal as the counter electrode, Celgard 2400 membrane as the separator and LiPF6 (1 M) in ethylene carbonate/dimethyl carbonate (EC/DMC, 1:1 vol%) as the electrolyte. The galvanostatic charge-discharge tests were carried out on a Land Battery Measurement System (Land, China). Cyclic voltammetry (CV) and impedance were performed using a VMP3 Electrochemical Workstation (Bio-logic Inc.).

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Acknowledgments

This work is financially supported by 100 Talents Programme of The Chinese Academy of Sciences, National Program on Key Basic Research Project of China (973 Program, Grant No. 2012CB215500), Foundation for Innovative Research Groups of the National Natural Science Foundation of China (Grant No. 20921002), National Natural Science Foundation of China (Grant No. 21101147), and the Jilin Province Science and Technology Development Program (Grant No. 20100102 and 20116008).

Author contributions

H.G.W. carried out the experiments and wrote the paper. D.L.M., Y.H. and X.L.H. conducted the SEM and TEM measurements. X.B.Z. supervised the research and revised the manuscript. All authors reviewed the manuscript.

Additional information

Supplementary information accompanies this paper at http://www.nature.com/ scientificreports

Competing financial interests: The authors declare no competing financial interests. License: This work is licensed under a Creative Commons

Attribution-NonCommercial-NoDerivative Works 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ How to cite this article: Wang, H., Ma, D., Huang, X., Huang, Y. & Zhang, X. General and Controllable Synthesis Strategy of Metal Oxide/TiO2 Hierarchical Heterostructures with Improved Lithium-Ion Battery Performance. Sci. Rep. 2, 701; DOI:10.1038/srep00701 (2012).