J. Phys. Chem. C 2010, 114, 8675–8678
8675
Fast Synthesis of r-MoO3 Nanorods with Controlled Aspect Ratios and Their Enhanced Lithium Storage Capabilities Jun Song Chen,† Yan Ling Cheah,‡ Srinivasan Madhavi,‡ and Xiong Wen Lou*,† School of Chemical and Biomedical Engineering, Nanyang Technological UniVersity, 70 Nanyang DriVe, Singapore 637457, and School of Materials Science and Engineering, Nanyang Technological UniVersity, 50 Nanyang AVenue, Singapore 639798 ReceiVed: February 26, 2010; ReVised Manuscript ReceiVed: April 2, 2010
Uniform R-MoO3 nanorods are synthesized with controlled aspect ratios through a fast hydrothermal route. The control over the aspect ratio of these as-prepared nanorods is realized by applying different reaction times of 2-20 h. Specifically, the nanorods prepared with a reaction time of 2 h are, on average, much shorter in length and slightly smaller in width compared with those obtained with a longer reaction time of 20 h. The products are thoroughly characterized by FESEM/TEM/XRD/BET techniques. The electrochemical properties of the samples are analyzed using cyclic voltammetry and charge-discharge cycling. These studies reveal that the as-prepared nanorods with a smaller aspect ratio exhibit a higher initial discharge capacity, a lower irreversible loss, and better rate behavior at different charge-discharge rates. When compared to R-MoO3 submicrometer particles prepared through direct thermal decomposition, these as-prepared nanorods show much better lihtium storage properties, demonstrating that enhanced physical and/or chemical properties can be obained from proper nanostructuring of the material. Introduction VI
V
Owing to its layered structure and the ease of the Mo /Mo couple, R-MoO3 has received much attention in the fields of electrochemical devices and others, such as capacitors, electrochemical energy storage devices, sensors, and electrochromic and photochromic devices.1,2 R-MoO3 possesses an anisotropic layered crystal structure that is composed of bilayers of distorted MoO6 octahedra in the [010] direction. The interlayer interaction is mainly through the van der Waals force.3,4 Such a layered crystal structure highly favors the reversible insertion/extraction of Li+ ions into/from the host R-MoO3 matrix.5 To date, R-MoO3 has been engineered into different morphologies, including nanowires,6 nanobelts,7-10 micro/nanorods,11,12 nanoparticles,13 and thin films.14-16 Among them, rod- or beltlike R-MoO3 has attracted particular interest due to its singlecrystalline nature that serves well as the representation of the layered structure and anisotropic growth. This category of onedimensional structure is believed to possess special characteristics over bulk crystals. Previously, we were among the first to synthesize R-MoO3 nanorods/ribbons via acidification of ammonium heptamolybdate tetrahydrate11 and further demonstrated the controlled growth of R-MoO3 nanorods along the [001] direction by introducing anatase TiO2 “caps”, allowing the fabrication of complex R-MoO3 nanostructures.17 However, the synthesis of pure R-MoO3 nanorods with controlled aspect ratios has hardly been achieved until now. Herein, we report the fast synthesis of R-MoO3 nanorods with controlled aspect ratios based on a modified hydrothermal method.11 The synthesis in the present study is more efficient by considering that pure R-MoO3 nanorods can be formed with a reaction time of only 2 h compared with 30-40 h in the original report. More * To whom correspondence should be addressed. Tel: (+65) 6316 8879. E-mail:
[email protected]. † School of Chemical and Biomedical Engineering. ‡ School of Materials Science and Engineering.
importantly, the aspect ratio of the synthesized R-MoO3 nanorods can be controlled by simply adjusting the reaction time. Specifically, the R-MoO3 nanorods formed with a reaction time of 2 h (designated as sample I) are, on average, much shorter in length and slightly smaller in width compared with the counterparts synthesized with a reaction time of 20 h (designated as sample II). Moreover, we evaluated their lithium storage capabilities. It is found that sample I with a smaller aspect ratio exhibits a higher initial capacity, a lower irreversible loss, and better rate behavior compared with the longer nanorods. Furthermore, these pure-phase nanorods manifest significantly enhanced lithium-ion battery performance than R-MoO3 submicrometer particles (designated as sample III) obtained by direct thermal decomposition of the precursor in air, which evidently shows that proper nanstructuring will lead to better physical and/or chemical properties of the material.18,19 Experimental Section Materials Preparation. The synthesis of R-MoO3 nanorods is based on a modified hydrothermal method.11 In a typical synthesis, 0.5-1.67 g of ammonium heptamolybdate tetrahydrate (AHM, (NH4)6Mo7O24 · 4H2O, Sigma-Aldrich) was dissolved in a mixed solution of 65% HNO3 and deionized H2O with a volume ratio of 1:5. After fully dissolved, this reaction solution was transferred into a Teflon-lined stainless steel autoclave (60 mL capacity) and heated at 160-200 °C in a preheated electric oven for 2-20 h. After cooling, the light gray product was harvested by centrifugation and washed thoroughly with ultrapure water before drying at 60 °C overnight. The R-MoO3 submicrometer particles were obtained by direct thermal decomposition of AHM at 400 °C for 2 h in air. Materials Characterization. Phase characterization was done by X-ray diffraction (XRD, Bruker, D8 - Advance X-ray Diffractometer, Cu KR, λ ) 1.5406 Å). The surface morphology of the synthesized products was examined using field emission scanning electron microscopy (FESEM, JEOL, JSM-6700F, 5
10.1021/jp1017482 2010 American Chemical Society Published on Web 04/09/2010
8676
J. Phys. Chem. C, Vol. 114, No. 18, 2010
Chen et al.
Figure 1. XRD patterns of samples I-III.
kV) and transmission electron microscopy (TEM, JEOL, JEM2010F, 200 kV). The N2 adsorption and desorption isotherm was obtained using a Quantachrome Instruments Autosorb AS6B. Electrochemical Measurement. The lithium insersion/ extraction tests were carried out using two-electrode Swageloktype cells with pure lithium as the counter and also the reference electrodes at room temperature. The working electrode was made of a mixture containing the active material (e.g., R-MoO3 nanorods with different aspect ratios), conducting carbon black (Super-P-Li), and an organic binder (poly(vinylidene difluoride)) in a weight ratio of 70:20:10. The electrolyte used was 1 M LiPF6 in a 50:50 w/w mixture of ethylene carbonate and diethyl carbonate. Cells were assembled in an argon-filled glovebox with the moisture and oxygen contents maintained below 1 ppm. The CV and electrochemical charge/discharge measurements were performed using a CHI electrochemical workstation and a NEWARE battery tester, respectively. The cells were charged and discharged at various current rates, and the voltage window was fixed between 1.5 and 3.5 V. Results and Discussion Figure 1 shows the X-ray diffraction (XRD) patterns of samples I-III. All the identified peaks for the as-prepared samples can be indexed to the orthorhombic R-MoO3 (JCPDS card no. 05-0508, S.G. Pbnm, ao ) 3.962 Å, bo ) 13.858 Å, co ) 3.697 Å). The intensities of the (020), (040), and (060) diffraction peaks of samples I and II are particularly high compared with that of sample III, suggesting a highly anisotropic growth.7 On the other hand, the pattern of sample III, which is in irregualr particle form, shows all other non-(0l0) peaks that are consistent with the standard XRD pattern (JCPDS card no. 05-0508) of an R-MoO3 powder sample. It can also be observed from the patterns that sample II shows much higher peak intensities, indicating better crystallinity. Peaks due to other phases are not identified in all samples, indicating high purity of the as-synthesized R-MoO3 nanorods and particles. The morphology of the synthesized R-MoO3 nanorods and the submicrometer particles was examined using field emission scanning electron microscopy (FESEM), with the results shown in Figure 2. It can be observed that the average size of the nanorods in sample I is ∼100 nm in width and 1-2 µm in length, whereas that in sample II increases to ∼200 nm in width and more than 10 µm in length (compare Figure 2, panels A and C). Thus, the aspect ratio (defined as length along [001] to width along [100]: L[001]/W[100])11 of sample I can be estimated to be in the range of 10-20, whereas that of sample II is more than 100. A closer examination reveals that the surface of the nanorods in sample I is quite rough, consisting of small
Figure 2. FESEM images of sample I (A, B), sample II (C, D), and sample III (E, F).
nanoparticles (Figure 2B), indicating that the crystal growth is still in its initial stage with a relatively poor crystallinity, which is in agreement with the above XRD analysis. With a longer reaction time, the nanorods in sample II have a rather smooth surface texture (Figure 2D). The R-MoO3 submicrometer particles obtained from direct thermal decomposition show very irregular shapes (Figure 2E,F). They are generally of several hundred nanometers in size, which is comparable to the width of the nanorods. The above observations clearly reveal the important effect of reaction time upon the morphology of the as-prepared nanorods. Previously, it has been determined that the R-MoO3 nanorods are formed from the dissolution of an intermediate hydrated crystal, and the crystal growth rates along different directions are in the following order: [001] > [100] > [010].11 In the present synthesis, pure R-MoO3 nanorods are already formed after only 2 h of hydrothermal treatment with a relatively small aspect ratio (Figure 2A,B) because of the much higher acid concentration used. However, there should still exist lots of Mo-containing species in the solution. With a prolonged reaction time, the dominant growth along the preferred [001] direction leads to a much larger aspect ratio.11 The surface area measurement by the Brunauer-Emmett-Teller (BET) method shows that sample I has the highest specific surface area of 15.2 m2 g-1, which is probably due to its surface roughness and defects, as well as its much smaller crystallite size. Sample II exhibits a lower surface area of about 12 m2 g-1, and the R-MoO3 submicrometer particles only has a surface area of 7.6 m2 g-1. In good accordance with the above SEM results, it can be observed from the transmission electron microscopy (TEM) images (Figure 3A,D) that the as-synthesized R-MoO3 nanorods with different aspect ratios have a uniform width along the whole length. From the high-magnification images (Figure 3B,E), the surface texture of sample I can be clearly observed to be quite rough, reflecting the small particles on the surface as identified under FESEM. Sample II exhibits a much smoother surface structure. The selected area electron diffraction (SAED, the insets in Figure 3C,F) patterns can be indexed to the [010] zone, which is consistent with the observation that these ribbon-like
R-MoO3 Nanorods with Controlled Aspect Ratios
J. Phys. Chem. C, Vol. 114, No. 18, 2010 8677
Figure 3. TEM images of sample I (A-C) and sample II (D-F). The insets in (B) and (E) are the corresponding low-magnification images. The insets in (C) and (F) are the SAED patterns of the corresponding regions marked by white circles.
R-MoO3 nanorods sediment mainly on their largest (010) plane during the preparation of the TEM specimen.11 It is thus illustrated that the growth direction is [001].9 It can also be observed from Figure 3F that there are two tiers of (010) planes in the nanorod (also indicated by darker contrast), which is due to the common on-plane growth of this type of ribbon-like R-MoO3 nanorod.11,17 We further evaluate the reversible lithium storage capabilities of the as-prepared R-MoO3 nanorods and submicroparticles. Figure 4 shows the representative cyclic voltammograms (CVs) of the as-prepared samples between 1.5 and 3.5 V. Consistent with the literature,12 one prominent pair of current peaks appears at 2.2 V (cathodic sweep) and 2.7 V (anodic sweep), marking its dominant effect during the phase-transition process of lithium insertion and deinsertion. Another peak can also be observed at 2.6 V during the first cathodic scan, and its position shifts to higher voltages in the subsequent scans, accompanied by decreasing intensity until complete disappearance in the fifth cycle. This indicates some irreversible lithium insertion into the crystal structure, which is believed to be caused by unrecoverable phase transformation, leading to irreversible capacity loss.12 With a closer look at Figure 4C, one can observe that the current of the dominant peaks of sample III decreases significantly in the fifth cycle with a subtantial shift in potential, indicating that the poorly crystallized sample III may not be able to efficiently intercalate lithium ions during insertion/deinsertion, resulting in severe capacity loss upon extended cycling. Figure 5A displays the charge-discharge voltage profiles of the three samples during the first cycle. In good agreement with the CV analysis, two plateau regions are present in the discharge curves. The first small plateau appears at about 2.7 V, which corresponds to the irreversible process; the second, more prominent, plateau occurs at 2.2-2.4 V, representing the dominant lithium intercalation process. At the end of the first discharge, a capacity as high as 294 mA h g-1 can be delivered by sample I, whereas that of sample II is only 212 mA h g-1. The subsequent charge capacities of 249 and 156 mA h g-1 can be obtained, leading to irreversible capacity losses of 15.3% and 26.4%, for samples I and II, respectively. It is thus evident that sample I with a smaller size demonstrates a higher initial capacity as well as a much lower irreversible loss, which could be attributed to its much shorter diffusion length, allowing more efficient insertion/deinsertion of lithium ions.18,19 Moreover, sample I has the highest specific surface area, which may give rise to more lithium insertion sites during charge-discharge
Figure 4. Representative cyclic voltammograms of sample I (A), sample II (B), and sample III (C). Analysis was preformed with a scan rate of 0.5 mV s-1 from 1.5 to 3.5 V.
cycles, contributing to its better lithium storage properties. Nevertheless, further investigations are clearly required to uncover other possible origins besides surface area and crystallinity, for example, surface chemistry, that are resposible for the advantageous electrochemical properties of sample I. Even though sample III is able to give a discharge capacity as high as 300 mA h g-1, it only delivers a reverible charge capacity of 155 mA h g-1, which leads to a much higher irreversible capacity loss of 48.3%. Figure 5B shows the cycling behavior of the as-prepared samples at different charge-discharge rates up to 400 mA g-1. Although a drop in capacity is observed for both sample I and II in the second cycle, sample I can deliver a higher capacity of 280 mA h g-1 compared with 200 mA h g-1 for sample II at a rate of 50 mA g-1. With the gradually increasing current rate, the capacity drops steadily to 270, 240, and 150 mA h g-1 for sample I and to 145, 130, and 120 mA h g-1 for sample II. It is apparent that sample I has a much better lithium storage capability than that of sample II. Sample III again shows the highest initial discharge capacity of 446 mA h g-1, which quickly drops to 120 mA h g-1 after only 10 cycles. As a result, sample III conveys a much lower capacity at every current rate. Figure 5C illustrates cycling behaviors of the three samples at 300 mA g-1. The capacity delivered by sample I drops steadily during the first 50 cycles and then levels off at about 161 mA h g-1. Sample II gives a lower initial capacity of 211
8678
J. Phys. Chem. C, Vol. 114, No. 18, 2010
Chen et al. poor capacity retention of sample III was also indicated by the above CV analysis. Conclusions In summary, a modified hydrothermal method is developed for the fast, high-yield synthesis of highly crystalline and phasepure R-MoO3 nanorods with controlled aspect ratios. The aspect ratio of the as-prepared R-MoO3 nanorods can be tuned by adjusting the reaction time. With a short reaction duration of 2 h only, the synthesized R-MoO3 nanorods have an aspect ratio of 10-20, which grows to more than 100 if the reaction is prolonged to 20 h. Furthermore, the lithium storage capabilities of the synthesized R-MoO3 nanorods are evaluated. The R-MoO3 nanorods with a smaller aspect ratio manifest a higher initial capacity, a lower initial irreversible loss, as well as better rate behavior compared with the sample with a larger aspect ratio. Our investigation also demonstrates that these as-prepared nanorods manifest a much enhanced lithium-ion battery performance compared with R-MoO3 submicrometer particles. The vast difference in lithium storage capabilities is believed to arise from differences in size, crystallinity, and possibly surface chemistry of the samples, which, together with the insertion/extraction mechanism, clearly require further in-depth investigations. Acknowledgment. We are grateful to Nanyang Technological University for financial support through the Start-Up Grant (SUG). References and Notes
Figure 5. First-cycle charge-discharge voltage profiles at 300 mA g-1 (A), rate behavior at various current rates (B), and cycling performance (C) at a constant current rate of 300 mA g-1 of samples I-III. Measurements were performed with a voltage window of 1.5-3.5 V. The discharge capacities (lithium insertion) are shown in both (B) and (C).
mA h g-1, then drops slightly to 133 mA h g-1 after 20 cycles, followed by an increase in capacity to 154 mA h g-1 after 50 cycles. Afterward, its capacity drops gradually to a value of 110 mA h g-1. This type of “wavelike” cycling behavior, which could probably be attributed to some long-term activation process in the composite electrode during charge-discharge cycling, has been reported in the literature9,20 and also observed by us for other metal oxides. This may be associated with partial crystallinity degradation of the electrode material to a disordered or amorphous-like structure during the initial cycling. As a result, more insertion sites could be made available and accessible for additional lithium storage, leading to an increase in the specific capacity. However, such newly created microstructures cannot be stable for many cycles, eventually leading to the normal capacity fading over extended cycling. Sample III produces the highest initial capacity of 300 mA h g-1 among the three samples. It decreases dramatically to 163 mA h g-1 in the second cycle, revealing a huge irreversible loss up to 45.7%, then falls and levels off at 53 mA h g-1 after 40 cycles. This
(1) Comini, E.; Yubao, L.; Brando, Y.; Sberveglieri, G. Chem. Phys. Lett. 2005, 407, 368. (2) Fang, L.; Shu, Y. Y.; Wang, A. Q.; Zhang, T. J. Phys. Chem. C 2007, 111, 2401. (3) Michalak, A.; Hermann, K.; Witko, M. Surf. Sci. 1996, 366, 323. (4) Tokarz-Sobieraj, R.; Hermann, K.; Witko, M.; Blume, A.; Mestl, G.; Schlogl, R. Surf. Sci. 2001, 489, 107. (5) Julien, C.; Khelfa, A.; Guesdon, J. P.; Gorenstein, A. Appl. Phys. A: Mater. Sci. Process. 1994, 59, 173. (6) Qi, B. G.; Ni, X. M.; Li, D. G.; Zheng, H. G. Chem. Lett. 2008, 37, 336. (7) Reddy, C. V. S.; Deng, Z. R.; Zhu, Q. Y.; Dai, Y.; Zhou, J.; Chen, W.; Mho, S. I. Appl. Phys. A: Mater. Sci. Process. 2007, 89, 995. (8) Reddy, C. V. S.; Walker, E. H.; Wen, C.; Mho, S. I. J. Power Sources 2008, 183, 330. (9) Mai, L. Q.; Hu, B.; Chen, W.; Qi, Y. Y.; Lao, C. S.; Yang, R. S.; Dai, Y.; Wang, Z. L. AdV. Mater. 2007, 19, 3712. (10) Tsumura, T.; Inagaki, M. Solid State Ionics 1997, 104, 183. (11) Lou, X. W.; Zeng, H. C. Chem. Mater. 2002, 14, 4781. (12) Li, W. Y.; Cheng, F. Y.; Tao, Z. L.; Chen, J. J. Phys. Chem. B 2006, 110, 119. (13) Lee, S. H.; Kim, Y. H.; Deshpande, R.; Parilla, P. A.; Whitney, E.; Gillaspie, D. T.; Jones, K. M.; Mahan, A. H.; Zhang, S. B.; Dillon, A. C. AdV. Mater. 2008, 20, 3627. (14) Julien, C.; Nazri, G. A.; Guesdon, J. P.; Gorenstein, A.; Khelfa, A.; Hussain, O. M. Solid State Ionics 1994, 73, 319. (15) Swiatowska-Mrowiecka, J.; de Diesbach, S.; Maurice, V.; Zanna, S.; Klein, L.; Briand, E.; Vickridge, I.; Marcus, P. J. Phys. Chem. C 2008, 112, 11050. (16) Iriyama, Y.; Abe, T.; Inaba, M.; Ogumi, Z. 12th International Conference on Solid State Ionics, Halkidiki, Greece, June 6-12, 1999; p 95. (17) Lou, X. W.; Zeng, H. C. J. Am. Chem. Soc. 2003, 125, 2697. (18) Wang, Y.; Cao, G. Z. AdV. Mater. 2008, 20, 2251. (19) Lou, X. W.; Archer, L. A.; Yang, Z. C. AdV. Mater. 2008, 20, 3987. (20) Julien, C.; Nazri, G. A. Solid State Ionics 1994, 68, 111.
JP1017482
