Article pubs.acs.org/Langmuir
Microwave-Induced Synthesis of Porous Single-Crystal-Like TiO2 with Excellent Lithium Storage Properties Dieqing Zhang,* Meicheng Wen, Peng Zhang, Jian Zhu, Guisheng Li,* and Hexing Li Key Laboratory of Resource Chemistry of Ministry of Education and Shanghai Key Laboratory of Rare Earth Functional Materials, Shanghai Normal University, 200234, People's Republic of China ABSTRACT: Porous anatase TiO2 single crystal architectures with large specific surface area and remarkable crystalline phase-stability were fabricated via a green microwave-assisted process. Ionic liquid was chosen as both an essential structuredirecting agent for the formation of the {001} facets exposed TiO2 and an etching agent source for selective erosion of the exposed {001} facets, leading to robust porous framework with exposed {101} facets. These porous anatase single crystals were thermally stable up to 800 °C, indicating excellent structure stability. The product showed stable cyclability at high current rate, better reversibility, and high Coulumbic efficiency of 100% for lithium storage.
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INTRODUCTION Many natural materials are composed with an inorganic framework. It contains cavities, cages, or tunnels in which water, carbon dioxide, and inorganic cations are turned into oxygen, sugar, and other important substances. Designing hierarchical porous functional materials appears today as a strong and competitive field of research.1 The porous structure can provide interconnected framework, hence generating high surface area, and improving the low diffusion kinetics. Particularly, mainly due to high surface area, chemical inertness, and cost, porous titanium dioxide (TiO2)-based materials are attractive candidates as adsorbents, catalysts, and electrodes for batteries.2 The templating method has been proven as an efficient route for the fabrication of porous or hollow TiO2 nanostructures.3−5 Nevertheless, such a method, requiring both the utilization of soft templates including polymers and the removal of templates, is usually very complicated and costly to produce. Therefore, designing porous TiO2 via a rapid, reproducible, and simple method remains a significant challenge. Using the lithium ion battery as a secondary battery is an efficient way to solve the energy crisis problems due to its high power and energy density, low pollution, high rate, low selfdischarge, and long cycle life.6 Among the polymorphs of TiO2, anatase is considered as the most promising candidate, serving as the anode material for Li-ion batteries, due to its low cost, abundance and nontoxicity.7 The mesoporous TiO2 electrode materials show great advantages, such as good contact with electrolyte, high specific surface area and improved Li+ permeation.8,9 However, the majority of these mesoporous TiO2 materials feature only a semicrystalline or even amorphous pore wall structure which makes them unsuitable for real application. The structure of mesoporous TiO2 might be unstable and cannot withstand the high-rate insertion/ extraction of lithium ions over extended cycling.10,11 Therefore, © 2012 American Chemical Society
TiO2 single-crystal-like materials are often highly required owing to their excellent structural durability. Nevertheless, they suffered from either limited reaction kinetics, inhomogeneous current distribution from surface to bulk, or inferior interfacial contact with electrolyte.12 Therefore, the design of singlecrystal-like TiO2 with honeycomb-like pores would be highly desirable for the large surface area, robust structure, and special morphologies. Such architecture will be favorable for shortening the diffusion length of ion transport and reducing volumetric change; and it is expected to withstand high-rate insertion/extraction. Herein, single-crystal like anatase TiO2 with “honeycomb” porous structure was fabricated by a green microwave-assisted approach, involving titanium tetrachloride aqueous solution and ionic liquid (1-methyl-imidazolium tetrafluoroborate) without using templates. As a green, simple, and efficient fabrication process, it allows the large-scale production of porous singlecrystal like anatase TiO2. Meanwhile, a high surface area (70 m2 g−1) was obtained, though the anatase TiO2 owned a singlecrystal-like structure. These porous anatase single crystals were thermally stable up to 800 °C, indicating excellent structure stability. Such unique properties resulted in excellent electrochemical performances, including stable cyclability at high current rates, better reversibility, and extremely high Coulumbic efficiency of nearly 100% for lithium storage.
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EXPERIMENTAL SECTION
Materials Preparation. In a typical synthesis, titanium tetrachloride (ACROS) aqueous solution (denoted as solution A) was prepared by dissolving 5.0 mL titanium tetrachloride in 15 mL of D.I. water, under the assistance of liquid nitrogen. Then 0.5 mL of ionic Received: December 21, 2011 Revised: February 10, 2012 Published: February 13, 2012 4543
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liquid 1-methyl-imidazolium tetrafluoroborate, synthesized according to literature,13 was added to the titanium tetrachloride aqueous solution containing 48 mL H2O and 2 mL Solution A. The mixture was sealed in a Teflon-lined-walled digestion vessel. After treatment at a controllable temperature of 150 °C for 90 min using a microwave digestion system (Ethos TC, Milestone), the vessel was then cooled down to room temperature. The samples were washed with deionized water and absolute ethanol, and dried in a vacuum at 80 °C for 4 h. Electrochemical Measurements. Electrochemical tests for the samples were carried out using coin cells (R2016) with Li metal foil as the counter electrode at room temperature. The working electrode consisted of 75 wt % active material (TiO2), 15 wt % conductive agent (carbon black), and 10 wt % polymer binder (PVDF). The electrolyte used was 1.0 M LiPF6 in a 1:1:1 (v/v) mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC). The coin cells were assembled in an Ar-filled glovebox with concentrations of moisture and oxygen below 1.0 ppm. Charge− discharge performance of the cells was evaluated using a battery test system (LAND CT2001 A model) at different current rates with a voltage window of 1−3 V. The cyclic voltammetry (1−3 V, 0.2 mV/s) of the cells was performed on an electrochemical workstation (CHI 750A). Materials Characterization. The crystallographic information of the samples were determined by X-ray diffraction (XRD, D/MAX2000 with Cu Kα1 irradiation). The morphology of the products were investigated by scanning electron microscopy (SEM, JEOL JSM6380LV) and transmission electronic micrograph (TEM, JEM-2010, operated at 200 kV), N2 adsorption−desorption (Quantachrome NOVA 4000e, at 77 K). On the basis of the adsorption branches of N2 sorption isotherms, the Brunauer−Emmett−Teller (BET) method was used to calculate the specific surface area (SBET) and the Barrett− Joyner−Halenda (BJH) model was used to calculate pore volume (VP), and the pore size distribution.
Figure 2. FESEM images (a,b), TEM images (c,d) of the porous TiO2 single crystals obtained after 90 min microwave irradiation.
as-prepared anatase TiO2 powders. The low-magnification (FESEM) image (Figure 2a) shows the high-yield synthesis of nanospheres with an average diameter of 700 nm. Close inspection shows an interesting honeycomb-like nanosphere with open porous structure (Figure 2b). The corresponding transmission electron microscopy (TEM) images (Figure 2c,d) show a core−corona architecture where the 600-nm-diameter core is much denser than the corona, indicating the formation of dense core and porous surface. To understand the formation mechanism of such a single crystal-like porous TiO2 nanohoneycombs, the morphologies of the intermediate products in different stages of evolution were examined by FESEM. As shown in Figure 3a, ball-like particles with an average diameter of less than 500 nm were observed immediately after
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RESULTS AND DISCUSSION Figure 1 shows the XRD pattern of the as-prepared sample synthesized at 150 °C under microwave irradiation for 90 min.
Figure 1. XRD pattern of the TiO2 samples synthesized at 150 °C under 90 min microwave irradiation.
Figure 3. FESEM images of the TiO2 samples synthesized at 150 °C under (a) 1 min, (b) 30 min, (c) 60 min, and (d) 90 min microwave irradiation.
The profile of the TiO2 is consistent with the standard diffraction pattern of bulk anatase TiO2, exhibiting (101), (103), (004), (112), (200), (105), (211), (213), (204), (116), (220), (107), (215), and (301) signals. It is attributed to TiO2 crystals with the tetragonal anatase phase (JCPDS: 21−1272). The narrow peak of the (101) with intense reflection suggests a high crystallinity of the sample. Figure 2a,b shows the fieldemission scanning electron microscopy (FESEM) images of the
1 min microwave irradiation. The XRD results (Figure 4) of the sample obtained after 1 min show that it possesses a high crystal degree of anatase. This indicates that the microwaveassisted approach is a facile way for preparing TiO2 crystals. After 60 min, (001) facets of anatase TiO2 single crystals can be observed as shown in Figure 5a. The selected area electron 4544
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Figure 6. Nitrogen adsorption−desorption isotherms and the pore size distribution calculated using the BJH method (inset) of the asprepared porous TiO2 nanohoneycombs.
Figure 4. XRD patterns of the TiO2 samples synthesized at 150 °C after (a) 1 min, (b) 30 min, (c) 60 min, and (d) 90 min microwave irradiation.
surface area and the pore volume were determined to be 70 m2 g−1 and 0.25 cm3 g−1. Such unique structure, with high specific surface area, large pore volume, and robust structure, may be favorable for the energy storage applications.15 It should be pointed out that these porous anatase TiO2 honeycombs are thermally stable up to 800 °C (Figure 7),
diffraction pattern (Figure 5b) further confirms diffraction spots of the [001] zone.14 The high-resolution TEM image (HRTEM) (Figure 5c) shows the (200) and (020) plane with a lattice spacing of 0.189 nm and an interfacial angle of 90 o. The SAED pattern recorded along the [111] axis (Figure 5d) of the sample displays the (101) and (011) facets with the interfacial angle of 82.4°. These results indicate the single-crystalline nature of the sample. Moreover, an eroded surface was also observed. The severity of the erosion was found to be increased with increased microwave irradiation (90 min) as shown by Figure 3d. Almost all the {001} facets were eroded away after 90 min of microwave irradiation, leading to the formation of hollow single-crystal aggregates with eight {101} facets remained as the frame. The current study shows that the formation of TiO2 nanohoneycombs can be divided into three steps: (1) the formation of nanosphere-like aggregates by assembled nanoparticles; (2) the formation of the multifaceted aggregates composed of anatase TiO2 single crystals with exposed {001} facets; (3) the selective erosion of the exposed {001}facets. During the reaction process, ionic liquid is believed to play a dual role: (1) to reduce the surface energy of {001} facets, (2) to act as a selective etching reagent of multifaceted aggregates. Such etching-induced porous structure could be confirmed by the corresponding nitrogen adsorption/desorption measurement, which indicated the presence of rather uniform nanoporous structure (Figure 6). The porous TiO2 nanohoneycombs have a broad pore size distribution ranging from 5.0 to 60 nm. This is in good agreement with FESEM and TEM observation. The Brunauer−Emmett−Teller (BET)
Figure 7. Wide-angle XRD patterns of the porous TiO2 nanohoneycombs calcined at different temperatures under air atmosphere for 1 h.
remaining the pure phase of anatase TiO2. This temperature is much higher than the phase-transfer temperature of 500 °C
Figure 5. SEM image (a), SAED pattern (b), and HRTEM image (c) recorded along the [001] axis and SAED pattern recorded along the [111] axis (d) of the sample obtained after 60 min microwave irradiation. 4545
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Figure 8. (a) Representative cyclic voltammograms of the sample at a scan rate of 0.2 mV s−1; (b) charge−discharge profiles at a current rate of 200 mA h g−1; and (c) cycling performance of the sample at various current rates.
from anatase to thermodynamically stable rutile for the traditional mesoporous TiO2. Such greatly enhanced thermal stability could be attributed to the single-crystal-like nature of the pore walls exposed with {101} facets. As known, the pore wall of the traditional porous TiO2 usually exists in a polycrystalline or even amorphous phase, producing an easier anatase-to-rutile phase transformation under high-temperature calcination compared to the single-crystal-like TiO2. Figure 8a shows the representative cyclic voltammograms (CV) of the obtained porous anatase TiO2 crystals at a scan rate of 0.2 mV s−1 in the potential range from 3.0 to 1.0 V. Two current peaks are observed at 1.7 and 2.1 V during the first cathodic and anodic scans. These peaks can be regarded as the signature of the lithium insertion/extraction processes in the anatase framework, which is consistent with the previous report.16 We further evaluated the lithium storage properties of the sample. Figure 8b shows the charge−discharge voltage profiles of the sample for the 1 st, 2 nd and 3 rd cycles at a current rate of 200 mA g−1. It includes three stages: the first stage of the quick voltage drop, the second stage of the distinct voltage plateau, and the third stage of a gradual decay in potential. Two distinct voltage plateaus could be observed at ∼1.7 and ∼2.1 V during the discharge and charge process, which is consistent with the above CV measurements. The insertion process gave a first discharge capacity of 156.1 mA h g−1 and the subsequent charge capacity of 155.8 mA h g−1, indicating a very low irreversible capacity loss of only 0.2% and a high Coulumbic efficiency of 99.8% for lithium storage in the first cycle. Generally, anatase TiO2-based electrodes exhibit an initial irreversible capacity loss (30−50%) in the first cycle.17
Such extremely low irreversible capacity loss (0.2%) in this work could break through the bottleneck problem of lithium storage. In the second cycle, the discharge and charge capacities are 150.6 and 150.6 mA h g−1, which result in a higher Coulumbic efficiency of 100%. The Coulumbic efficiency maintained 100% in the third cycles as shown by the discharge and charge capacities of 146.5 and 146.5 mA h g−1, indicating a good reversibility. The above results suggest that the intercalation of Li into the interstitial octahedral sites of the anatase phase of the sample reported here is highly reversible. In the field of lithium-ion batteries, it is worth mentioning that robust structures with smaller transport pathways are often desired. Such a low irreversible capacity loss might be partly attributed to the unique honeycomb nanostructure (a porous surface and a dense core) of anatase single crystals. Such porous core−corona architecture is thus beneficial for the electrochemical storage behavior because of a reduced transport length for Li+ ions.18 The high specific surface area also contributes the superior electrode−electrolyte contact area, facilitating the electrochemical performance. Figure 8c shows the cycling performance of the anatase TiO2 nanohoneycombs at different current rates. It is obvious that the as-prepared material demonstrates excellent cyclic capacity retention at each current rate during extended charge− discharge cycles. After 100 cycles, a reversible capacity of 114 mA h g−1 can be retained at a current rate of 200 mA g−1, which is much higher than that of anatase solid spheres (200 nm in diameter) with the low specific capacity of only ∼75 mA h g−1.19 When the current rate is increased to 1000 mA g−1, a capacity of 100 mA h g−1 can still be delivered after 100 cycles. 4546
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(6) Brezesinski, T.; Wang, J.; Polleux, J.; Dunn, B.; Tolbert, S. H. J. Am. Chem. Soc. 2009, 131, 1802−1809. (7) Liu, S. W.; Yu, J. G.; Jaroniec, M. Chem. Mater. 2011, 23, 4085− 4893. (8) Zhan, Y. Y.; Tang, Y. X.; Yin, S. Y.; Zeng, Z. Y.; Zhang, H.; Li, C. M.; Dong, Z. L.; Chen, Z.; Chen, X. D. Nanoscale 2011, 3, 4074−4077. (9) Haetge, J.; Hartmann, P.; Brezesinski, K.; Janek, J.; Brezesinski, T. Chem. Mater. 2011, 23, 4384−4393. (10) Ye, J. F.; Liu, W.; Cai, J. G.; Chen, S.; Zhao, X. W.; Zhou, H. H.; Qi, L. M. J. Am. Chem. Soc. 2011, 133, 933−940. (11) Chen, S. C.; Lin, H.; Qiao, S. Z.; Lou, X. W. J. Mater. Chem. 2011, 21, 5687−5692. (12) Lee, K. H.; Song, S. W. Appl. Mater. Interfaces 2011, 3, 3697− 3703. (13) Holbrey, J. D.; Seddon, K. R. Dalton Trans. 1999, 2133−2140. (14) Zhang, D. Q.; Li, G. S.; Yang, X. F.; Yu, J. C. Chem. Commun. 2009, 4381−4383. (15) Shin, J. Y.; Samuelis, D.; Maier, J. Adv. Funct. Mater. 2011, 21, 3464−3472. (16) Chen, J. S.; Tan, Y. L.; Li, C. M.; Cheah, Y. L.; Luan, D. Y.; Madhavi, S.; Boey, F. Y. C.; Archer, L. A.; Lou, X. W. J. Am. Chem. Soc. 2010, 132, 6124−6130. (17) Xu, J. W.; Ha, C. H.; Cao, B.; Zhang, W. F. Electrochim. Acta 2007, 52, 8044−8047. (18) Shi, Y. F.; Guo, B. K.; Corr, S. A.; Shi, Q. H.; Hu, Y. S.; Heier, K. R.; Chen, L. Q.; Seshadri, R.; Stucky, G. D. Nano Lett. 2009, 9, 4215− 4220. (19) Chen, J. S.; Lou, X. W. Electrochem. Commun. 2009, 11, 2332− 2335. (20) Chen, J. S.; Wang, Z. Y.; Dong, X. C.; Chen, P.; Lou, X. W. Nanoscale 2011, 3, 2158−2161. (21) Sun, C. H.; Yang, X. H.; Chen, J. S.; Li, Z.; Lou, X. W.; Li, C. Z.; Smith, S. C.; Lu, G. Q.; Yang, H. G. Chem. Commun. 2010, 46, 6129− 6131.
It is amazing that further increasing the current rate to 2000 mA g−1 leads to a slight decrease in capacity (from 100 to 87 mA h g−1) after 100 cycles. It can be comparable to that of the graphene-wrapped TiO2 hollow structures (∼90 mA h g−1) and much higher than that of pure TiO2 hollow structures (∼60 mA h g−1).20 Furthermore, the specific capacity and cyclability of the porous TiO2 crystals with exposing {101} facets retained at a current rate of 2000 mA g−1 is also better than that of the solid TiO2 crystals with exposing {101} facets.21 This result demonstrates that the porous single-crystal aggregates with exposed {101} facets possess a very stable framework, which allows alleviation of the structural damage caused by the insertion and extraction processes during cycling. These results imply that the lithium diffusion is highly efficient in the TiO2 single crystal aggregates. More importantly, such excellent rate performance may be partly due to the remarkable crystalline phase-stability, which ensures the crystal structure stability. In conclusion, anatase TiO2 nanohoneycombs with remarkable crystalline phase-stability, excellent cycling, and rate performance for lithium storage were synthesized via a green microwave-assisted process. Ionic liquid was chosen as both an essential structure-directing agent for the formation of the {001} facets exposed TiO2 and an etching agent source for selective erosion of the exposed {001} facets, leading to robust porous framework with exposed {101} facets. Such singlecrystal-like porous anatase TiO2 grants more lithium insertion sites, more space to buffer the volume change, and a reduced effective Li-ion diffusion distance during the extended charge− discharge process, due to its high surface area, large internal voids, and robust structure. The synthesis methodology explored in this study is energy-saving and cost-effective, and can be extended to synthesize other efficient electrodes for lithium storage.
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AUTHOR INFORMATION
Corresponding Author
*Tel: 86)-021 6432 2272; fax: (86)-021 6432 2272; e-mail:
[email protected] (D.Z.);
[email protected] (L.G.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to thank the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai Institutions of Higher Learning, the National Natural Science Foundation of China (21007040, 21047009, 20907032, 20825724), the Research Fund for the Doctoral Program of Higher Education (20103127120005), the Innovation Program of Shanghai Municipal Education Commission (12YZ079), the Pujiang Talents Programme (11PJ1407500), and the Project supported by the Shanghai Government (10160503200, 11ZR1426300, SK201104).
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REFERENCES
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