J. Phys. Chem. C 2007, 111, 1651-1657
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CeO2 Spherical Crystallites: Synthesis, Formation Mechanism, Size Control, and Electrochemical Property Study Fu Zhou,*,† Xuemei Zhao,‡ Hai Xu,‡ and Cunguang Yuan† Department of Chemistry and Center for Bioengineering and Biotechnology, China UniVersity of Petroleum, Qingdao, Shandong 266555, People’s Republic of China ReceiVed: September 15, 2006; In Final Form: NoVember 27, 2006
CeO2 spherical crystallites aggregated by small CeO2 nanoparticles in the diameter range of 5-10 nm were successfully fabricated through a facile surfactant-assisted hydrothermal method. PVP (poly(vinylpyrrolidone)) was applied as surfactant to facilitate the oriented aggregation of small CeO2 nanoparticles into spherical crystallites. The size of as-obtained CeO2 spherical crystallites could be deliberately controlled in the range 100-800 nm by varying synthetic parameters such as the molar ratio of PVP (repeating units) to Ce(NO3)3‚ 6H2O and the concentration of Ce(NO3)3‚6H2O solution. The formation mechanism was briefly discussed and the electrochemical properties of as-synthesized CeO2 spherical crystallites were studied by galvanostatic methods. The electrochemical test results show that the as-obtained CeO2 spherical crystallites have promising electrochemical properties that may enable them to be applied as an anode material in lithium ion batteries.
1. Introduction In past decades, extensive research has been carried out in the design and preparation of nanostructures with different shapes and sizes because of their corresponding novel properties and potential applications.1-3 Various methods have been put forward for the synthesis of diverse nanometer-scale materials.4-8 Recently, the preparation of CeO2 nanostructures has attracted extensive attention among synthetic chemists owing to their wide applications in fields such as polishing materials, catalysts, oxygen gas sensors, irradiation protectors, high-temperatureoxidation safeguards, and solid electrolytes for low-temperature solid oxide fuel cells (LTSOFC).9-19 Previous research has proved that nanocrystalline CeO2 has superior properties compared with its bulk counterparts. Chiang et al. demonstrated a 4 order of magnitude increase in electronic conductivity in CeO2 nanocrystals compared to micrometer-sized coarse grains.18 Due to the quantum size effect, a blue shift in the UV absorption spectrum was observed.20 The rate of CO oxidation on gold deposited on nanocrystalline particles of cerium dioxide was found to be 100 times higher than that of Au on regular CeO2 support.21 Thus, the development of preparation methods to control the size and morphology of CeO2 is needed for tapping the full potential of CeO2. Many synthetic methods have been applied to the preparation of CeO2 nanostructures, including spray pyrolysis,23 sonochemical and microwave assisted thermal decomposition,24 electrosynthesis,25 gas condensation,26 homogeneous precipitation,27 flux method28,29 and wet chemical methods.30-32 The synthesis of CeO2 nanoparticles33-35 and somenovelCeO2 structures,includingnanorods,36-38 nanowires,39-43 nanotubes,44,45 nanocubes,37,46 microplates,37,47 and other morphological structures48-50 have been reported by chemical scientists. However, the preparation of CeO2 spherical crystallites has rarely been reported. Yu et al. have recently reported * Corresponding author. Telephone: +86-532-86981562. Fax: +86532-86981318. E-mail:
[email protected]. † Department of Chemistry. ‡ Center for Bioengineering and Biotechnology.
the fabrication of CeO2 nanospheres from (NH4)2Ce(NO3)6 through a refluxing process in an ethylene glycol (EG) solution system.51 Caruso et al. have fabricated porous CeO2 spheres utilizing porous polymeric beads as templates through a nanoparticle infiltration process.52 However, to the best of our knowledge, the size-controllable hydrothermal synthesis of CeO2 spherical crystallites has rarely been reported. In recent years, research on lithium ion batteries has become the focus of chemists owing to their high energy density and high operative voltage, which lead to their wide application in portable electronics and high-technology devices. The process in the lithium cell involves simultaneous intercalation/deintercalation of Li ions in two electrode host materials, one at low potential (anode) and one at high potential (cathode). Thus, one of the main challenges in the design of lithium ion battery systems is to find suitable anode materials, which has evoked great interest among material researchers. Metallic lithium was first applied as an anode material in a lithium ion battery, but it was found to be very problematic. This is due to the continuous reactions between highly reactive lithium deposits (formed during the charging process of a lithium ion battery) and the solution components. Hence, major problems in rechargeable batteries based on lithium metal anodes are the loss of solution upon charge/discharge cycling that considerably limits the cycle life of these batteries and dendrite formation during Li deposition, which may short the batteries and thus create severe safety problems in their current use. Successful alternatives to lithium anodes in rechargeable batteries were found to be lithiated carbonaceous materials, mainly graphite.53-56 Indeed, the development of lithiated carbon anodes and lithiated transition metal oxide cathodes (e.g., LiMn2O4, LiCoO2, LiNiO2), both reversibly inserting lithium into nonaqueous electrolyte solutions, paved the way to the invention and commercialization of rechargeable lithium ion batteries. These batteries, which are now the mainstream of the lithium ion battery market, can indeed be considered as one of the most impressive successes of the electrochemistry technological community in recent years. However, although changing from lithium metal
10.1021/jp0660435 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/09/2007
1652 J. Phys. Chem. C, Vol. 111, No. 4, 2007 to lithiated graphite means a gain in stability, safety, and cycle life of rechargeable Li batteries, it is at the expense of loss of capacity (372 mA h g-1 for fully lithiated graphite, LiC6, compared with 3800 mA h g-1 for lithium metal). Therefore, there is a continuous driving force for the development of alternative anode materials, for which the capacity is much higher than that of lithiated graphite yet the safety features are acceptable (i.e., much better compared with metallic lithium). Many novel materials were proposed for potential application as future anode materials in lithium ion batteries, including lithium transition metal nitride,57,58 transition metal sulfide,59,60 tin oxide,61,62 tin-based alloys,63,64 and transition metal oxides. Among these potential alternatives, nanoscale transitional metal oxides were generally viewed as the most promising substitute for currently applied carbon-based anode materials owing to their high electrochemical capacities (about 700 mA h g-1), great capacity retention, and high recharging rates. Tarascon et al. have done a lot of work on the application of transition metal oxides as anode materials in lithium ion battery systems.65-67 Other electrochemistry scientists have also made great contributions to the electrochemical study of transition metal oxides.68-71 The outstanding electrochemical performances of transition metal oxides make them the most fascinating candidate for future lithium ion battery anode materials. Among all the transitional oxides, CeO2 is believed to be one of the most favorable candidates as an anode material for lithium ion battery systems owing to its outstanding superiority compared with other transitional metal oxides, such as large oxygen storage capacity, high thermal stability, facile electrical conductivity and diffusivity, and the quick and expedient mutation of the oxidation state of cerium between Ce(III) and Ce(IV). Herein, we report a facile surfactant-assisted hydrothermal synthetic route of CeO2 spherical crystallites. The formation mechanism is briefly discussed and the deliberate control of crystallite size is acquired by simply varying the synthetic parameters such as the molar ratio of surfactant poly(vinylpyrrolidone) (PVP) to Ce(NO3)3‚6H2O and the concentration of Ce(NO3)3‚6H2O solution. Galvanostatic tests were employed on as-obtained CeO2 spherical crystallites, indicating their promising features for application as anode materials in lithium ion batteries. 2. Experimental Section All the reagents were of analytical grade and were used without further purification. In a typical synthetic procedure of CeO2 spherical crystallites, 5 mmol of Ce(NO3)3‚6H2O and 10 mmol of PVP (repeating units) were dissolved in 40 mL of distilled water and stirred magnetically for 30 min to get good homogeneity. The resulting solution was transferred into a 50 mL Teflon-lined stainless steel autoclave. The autoclave was maintained at 140 °C for 24 h. After cooling to room temperature naturally, the light-yellow precipitates were filtered off, washed with distilled water and anhydrous alcohol several times, and dried in a vacuum at 80 °C for 12 h. X-ray diffraction patterns were recorded by using a Philiphs X’Pert Super diffractometer with graphite monochromatized Cu KR radiation (λ ) 1.541 78 Å) in the 2θ range of 10°-80°. The morphology of the products was examined by a scanning electron microscope (SEM) using an X-650 microanalyzer. FESEM images were taken on a field emission scanning electron microscope (JEOL JSM-6300F, 15 kV). Transmission electron microscopic (TEM) images were produced with a Hitachi 800 transmission electron microscope with the accelerating voltage of 200 kV. The samples used for characterization were dispersed
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Figure 1. XRD pattern of the typical sample.
in absolute ethanol and were ultrasonicated before SEM, FESEM, and TEM tests. Teflon cells were made to study the electrochemical properties of the samples. The positive electrode was composed of asobtained CeO2 spherical crystallites, carbon black, and poly(vinylidene fluoride) (PVDF) at a weight ratio of 80:10:10. The mixed slurry was uniformly cast on aluminum foil, dried in air at 140 °C for 24 h, pressed under 20 MPa pressure, and kept in a vacuum at 140 °C for 24 h to make the positive electrodes. Metallic lithium was used as the anode. The electrolyte was 1 M LiPF6 in ethylene carbonate and diethyl carbonate (EC/DEC 1:1 v/v). Celgard 2500 was used as the separator. The cells were assembled in an argon-filled glovebox in which both the moisture and the oxygen levels were less than 1 ppm. The electrochemical tests were made in the voltage range of 0.13.0 V at a current density of 0.1 C. Fifty charge/discharge cycles were processed to investigate the cycleability of the products. 3. Results and Discussion 3.1. Sample Characterization. X-ray powder diffraction (XRD) patterns reveal the phase and purity of as-obtained products. Figure 1 shows the XRD pattern of a typical CeO2 spherical crystallite sample fabricated by hydrothermal reaction using 5 mmol of cerium nitrate and 10 mmol of PVP (repeating units) as reagents at 140 °C. All the reflection peaks could be readily indexed to pure cubic fluorite CeO2 with a lattice constant a ) 5.418 Å (JCPDS Card No. 34-0394). The particle size of as-prepared CeO2 samples was estimated to be around 10 nm by the Debye-Scherrer formula. No impurity phases were detected from the XRD pattern, indicating that CeO2 spherical crystallites with high purity could be obtained under current synthetic conditions. The morphology and structure of the typical sample were investigated by SEM, FESEM, and TEM techniques. The panoramic morphologies of the typical sample were examined by scanning electron microscopy (SEM), and are shown in Figure 2A,B. The results indicate that the product consists of spherical crystallites with smooth surfaces in the size range of 500-600 nm. The proportion of spherical crystallites in the sample is above 90% and their yield is about 70% based on the original reagents. Careful observation of the typical sample surface was acquired by field emission scanning electron microscopy (FESEM), which revealed the accurate surface conditions of as-obtained CeO2 spherical crystallites. The FESEM images of the typical sample are given in Figure 2C,D, revealing that the surface of the as-obtained CeO2 spherical crystallite was not as smooth as shown in the low magnification SEM images. A clear grain boundary can be observed on the
CeO2 Spherical Crystallites
Figure 2. SEM and FESEM images of the typical sample: (A, B) low magnification SEM images; (C, D) high magnification FESEM images.
sample surface, indicating that as-obtained CeO2 spherical crystallites are constituted by the oriented aggregation of small CeO2 nanoparticles. The diameter of the compositive small CeO2 nanoparticles is in the range of 5-10 nm calculated from the dimension distribution of grain boundaries, which agrees well with the calculated particle size from the XRD pattern by the Debye-Scherrer formula. More details about the structure of as-obtained CeO2 spherical crystallites were investigated by the transmission electron microscopy (TEM), which are shown in Figure 3. Low magnification TEM images (Figure 3A,B) indicate that welldeveloped spherical CeO2 crystallites were obtained with the size range of 500-600 nm, which agrees well with the results of SEM tests. High magnification images (Figure 3C,D) reveal that the as-obtained CeO2 spherical crystallites are composed of small CeO2 nanoparticles with diameter of 5-10 nm, which validates the observation results of FESEM tests. The electron diffraction pattern of as-obtained CeO2 spherical crystallites is given in the inset of Figure 3C, which indicates the oriented aggregation of the nanoparticle building blocks in the sample. 3.2. Possible Formation Mechanism of the As-Obtained CeO2 Spherical Crystallites. To study the formation mechanism of as-obtained CeO2 spherical crystallites, the evolution process of the typical sample was examined thoroughly by detailed TEM tests. The synthetic process was ceased at definite reaction periods of 1, 6, and 12 h, and the as-obtained intermediate products were separated for TEM study. Although we knew that the reaction did not stop immediately after the autoclave was removed from the heater owing to heat transfer reasons, we did believe that the intermediate precipitates obtained represented certain stages in the formation process. Figure 4 shows the TEM images of as-obtained intermediate products. No products were obtained at the reaction time of 1 h. When the reaction time was prolonged to 6 h, a small amount
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Figure 3. TEM images of the typical sample: (A, B) low magnification; (C, D) high magnification. Inset in (C): SAED pattern of asobtained CeO2 spherical crystallites.
Figure 4. TEM images of intermediate products at different reaction periods: (A) 6 h; (B-D) 12 h.
of intermediate product could be obtained. The phase and purity of the intermediate products was studied by the XRD technique. All the reflection peaks in the XRD patterns (not shown here) of the intermediate products agreed well with those of pure cubic fluorite CeO2 (JCPDS Card No. 34-0394). The TEM image (Figure 4A) of the 6 h product reveals that the intermediate product was composed of loosely aggregated CeO2 nanoparticles with the diameter of 5-10 nm, which came into being at the
1654 J. Phys. Chem. C, Vol. 111, No. 4, 2007 initial stage of reaction. The aggregation of CeO2 nanoparticles was so loose in the 6 h intermediate product that large bright areas exist in the TEM image (indicated by the arrow), indicating the interspaces between the compositive CeO2 nanoparticles. It can also be observed that the intermediate product has no regular morphology and the size of the intermediate product was 100200 nm. The TEM images of the intermediate product at the reaction period of 12 h are shown in Figure 4B,C. As the reaction proceeded, the aggregation of CeO2 nanoparticles became more and more compact, as shown in Figure 4B. The interspaces between the compositive CeO2 nanoparticles almost disappeared in the 12 h intermediate product. The size of the aggregated CeO2 crystallites also increased to 200-300 nm. The development of regular morphology was detected in the product, which displayed nearly cubic morphology with slightly truncated corners and edges. Interestingly, we found some approximate core-shell structures in the TEM observation of the 12 h intermediate product, which is shown in Figure 4 C,D. The detection of such core-shell structures would be firm proof of the oriented aggregation process of CeO2 nanoparticles into as-obtained CeO2 spherical crystallites. The final product was obtained at the reaction time of 24 h, as shown in Figure 3. CeO2 crystallites with well-developed spherical morphology were successfully obtained, and the size increased finally to 500-600 nm. Based on the above experimental results, a possible oriented aggregation formation mechanism was proposed for our fabrication route of CeO2 spherical crystallites. Oriented aggregation was regarded as one of the most important crystal growth mechanisms, which led to the formation of multiform crystal structures and morphologies.72-77 In our synthetic process, small CeO2 nanoparticles appeared first in the solution under hydrothermal conditions. These newly formed nanoparticles had large specific surface area and high surface energy, which made them highly reactive. Thus, these small CeO2 nanoparticles tended to aggregate into bigger crystallites to reduce their specific surface area and surface energy in the following hydrothermal process. Therefore, under the assistance of surfactant, the oriented aggregation of initially formed CeO2 nanoparticles eventually led to the formation of CeO2 spherical crystallites. The application of surfactant PVP in our synthetic route played an important role in the formation of CeO2 spherical crystallite. It was generally believed that the building blocks for oriented aggregation were usually nanoparticles with surfaces stabilized by organic coating, and weakly protected nanoparticles often undergo entropy-driven random aggregation.78 PVP had been applied as an important surfactant for the synthesis of nanomaterials for a long time, and various nanostructures, includingnanowires,79-81 nanocables,82,83 nanoplates,84,85 nanocubes,86,87 and so on88-90 were successfully fabricated under the assistance of PVP. Although the exact function of PVP on the shape selectivity in our synthetic system was yet to be fully understood, it was believed that the selective adsorption of PVP on various crystallographic planes of CeO2 via the strong selective interaction between the surfaces of CeO2 nanoparticles and PVP through coordination bonding with the O and N atoms of the pyrrolidone ring played a major role in determining the product morphology. The strong interaction between CeO2 nanoparticles and PVP led to a relatively large amount of PVP residues in the final product, which was confirmed by thermogravimetric analysis (TGA). The TG curve (Figure 5) shows two main weight loss steps from room temperature to 600 °C. The 2% weight loss below 120 °C was attributed to the evaporation of residual water in the crystallites. In the heating temperature range
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Figure 5. TG curve of as-obtained CeO2 spherical crystallites.
Figure 6. SEM images of CeO2 samples using different surfactants: (A) CTAB; (B) PEG-10000.
of 120-400 °C, the weight loss could be attributed to the decomposition of residual PVP in the CeO2 spherical crystallites. Thus, the percentage of residual PVP in the final product was as high as 8%, which validated that a strong interaction existed between CeO2 nanoparticles and PVP. The typical sample after calcination at 400 °C for 1 h with a heating rate of 5 °C min-1 was also characterized by SEM and TEM tests. The observation results, which were not included in our paper, indicated that the size and morphology of the calcined samples were nearly identical with those of the typical sample before the calcination treatment. Comparative experiments were made to confirm the indispensable function of PVP in the formation of CeO2 spherical crystallites. In comparative experiment 1, no surfactant was used and other synthetic parameters were kept just the same as those in the typical synthetic procedure. Nearly no product was obtained without the assistance of surfactants. In comparative experiment 2, PVP was substituted by CTAB while other synthetic parameters were kept unchanged. The resulting product was mainly CeO2 crystallites with cubelike morphologies (Figure 6A), and the size of as-obtained CeO2 cubic crystallites increased greatly to 0.9-1 µm. In comparative experiment 3, PEG-10000 was used instead of PVP. Figure 6 B shows the SEM image of the product. No regular morphology could be observed, and the size also reached the micrometer range. The above results of comparative experiments further validated the important role of PVP in the formation of CeO2 spherical crystallites. The exact mechanism of the function of PVP in the formation of CeO2 spherical crystallites is still being researched in our laboratory and will be reported later.
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Figure 7. SEM images of CeO2 spherical crystallites synthesized at different molar ratios of PVP (repeating units) to Ce(NO3)3‚6H2O (Ce(NO3)3‚ 6H2O, 5mmol): (A) 4:1; (B) 2:1; (C) 1:1.
3.3. Size Control of the As-Obtained CeO2 Spherical Crystallites. The size of as-obtained CeO2 spherical crystallites could be deliberately controlled in our synthetic method simply by varying the synthetic parameters such as the molar ratio of PVP (repeating units) to Ce(NO3)3‚6H2O and the concentration of Ce(NO3)3‚6H2O precursor. For the study of the influence of the molar ratio of PVP to Ce(NO3)3‚6H2O on the size of CeO2 spherical crystallite, the molar ratio of PVP to Ce(NO3)3‚6H2O was increased from 1:1 to 4:1 while the amount of Ce(NO3)3‚ 6H2O stable at 5 mmol and other synthetic parameters were kept unchanged. SEM images of as-obtained samples are given in Figure 7, revealing that, concomitant with the increase of molar ratio, the size of obtained CeO2 spherical crystallites increased from 300-400 nm (molar ratio 1:1) to 600-800 nm (molar ratio 4:1). Similar size-control phenomena were also reported by other science workers in their synthesis of metal and metal oxide nanostructures.91,92 To study the influence of Ce(NO3)3‚6H2O solution concentration on the size of as-obtained CeO2 spherical crystallites, the amount of Ce(NO3)3‚6H2O was varied from 10 mmol to 1 mmol with the molar ratio of PVP to cerium nitrate kept at 1:1 and other synthetic parameters unchanged. SEM images of the products are shown in Figure 8, revealing that the size of asobtained CeO2 spherical crystallites decreased from 600-700 nm (10 mmol of Ce(NO3)3‚6H2O) to around 100 nm (1 mmol of Ce(NO3)3‚6H2O). The above results indicated that, by controlling the amount of Ce(NO3)3‚6H2O reagent, the size of as-obtained CeO2 spherical crystallites could be well controlled, similar to the size-control process of CeO2 nanocubes reported by Gao et al.93 Based on the experimental data obtained above, it could be concluded that a low molar ratio of PVP (repeating units) to Ce(NO3)3‚6H2O and low concentration of Ce(NO3)3‚6H2O reagent were contributive to the decrease of the size of as-obtained CeO2 spherical crystallites. The size of CeO2 spherical crystallites could be reduced to around 100 nm under the synthetic parameters of 1 mmol of Ce(NO3)3‚6H2O reagent and 1:1 molar ratio of PVP (repeating units) to Ce(NO3)3‚6H2O reagent. 3.4. Electrochemical Study of the As-Obtained CeO2 Spherical Crystallites. Cerium oxide (CeO2) has the fluorite structure with a face-centered-cubic lattice (Fm3m space group) that is stable from room temperature to its melting point, and the oxidation state of cerium in CeO2 can mutate quickly and expediently between Ce(III) and Ce(IV), leading to its wide application in solid-state fuel cells (SOFC), oxygen storage materials, superconductors, and so on. Based on these excellent physical and chemical properties, we speculated that CeO2 could be a promising anode material for lithium ion batteries. Jiang et al. had reported the lithium ion storage properties of CeO2
Figure 8. SEM images of CeO2 spherical crystallites synthesized with different amounts of Ce(NO3)3‚6H2O (molar ratio of PVP to Ce(NO3)3‚ 6H2O 1:1): (A) 10 mmol; (B) 5 mmol; (C) 2 mmol; (D) 1 mmol.
thin films and their possible application as anode materials for lithium ion batteries,94 but the capacity and cycleability of assynthesized CeO2 films were not very good, which limited their application as anode materials. The electrochemical properties of as-obtained CeO2 spherical crystallites were investigated by galvanostatic tests in the voltage range of 0.1-3.0 V with the current density of 0.1 C. Fifty charge/discharge cycles were applied to study the cycleability of as-obtained CeO2 spherical crystallite. Figure 9 gives the initial charge/discharge curve and the discharge capacity vs cycle number curve of the typical sample (Ce(NO3)3‚6H2O, 5 mmol; molar ratio of PVP (repeating units) to Ce(NO3)3‚6H2O, 2:1). The as-obtained CeO2 spherical crystallites showed an initial discharge capacity of about 460 mA h g-1, which was much higher than that of presently used carbonaceous anode materials (theoretical capacity 372 mA h g-1). The cycleability of asobtained CeO2 spherical crystallites was also very good. After 50 charge/discharge cycles, the discharge capacity still remained
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Figure 9. Initial charge/discharge curve (A) and cycling behavior (B) of the typical sample before and after calcination treatment (1, before calcination treatment; 2, after calcination treatment).
TABLE 1: Initial Discharge Capacity of As-Obtained CeO2 Spherical Crystallites with Different Sizes before and after Calcination Treatment at 400 °C initial discharge capacity (mA h g-1) crystallite size range (nm)
original sample
sample after calcination
600-800 500-600 300-400 200-300 100-150
400 460 520 600 680
480 550 610 700 820
at around 430 mA h g-1, indicating only 7% loss of the initial discharge capacity, which was far better than the results reported by Jiang et al. The sample after 50 charge/discharge cycles was characterized by the XRD technique. The XRD pattern, which was not included in our paper, showed that all the typical diffraction peaks of cubic fluorite CeO2 remained nearly unchanged and nearly no broadening of diffraction peaks could be observed, indicating that the fcc (face-centered cubic) crystal structure of as-obtained CeO2 spherical crystallites was well retained during the prolonged charge/discharge cycles. Galvanostatic tests were also applied to the typical sample after calcination at 400 °C for 1 h with a heating rate of 5 °C min-1 to get rid of the residual PVP in the product. The results indicated that the initial discharge capacity was increased greatly to 550 mA h g-1. After 50 charge and discharge cycles, the discharge capacity still remained at 530 mA h g-1, indicating that only 4% of the initial discharge capacity is lost. The reason for the great improvement in the electrochemical performances of as-obtained CeO2 spherical crystallites after calcination at 400 °C could be that, after calcination treatment, the residual PVP in the products was driven out, which led to the improvement of their electrochemical properties just like our former electrochemical research on spinel LiMn2O4 nanoparticles synthesized by a refluxing method.95 The excellent electrochemical properties of the CeO2 spherical crystallites could be explained by the following: (1) CeO2 can easily shift between CeO2 and Ce2O3 under oxidizing and reducing conditions, because of the quick and facile mutation in the redox couple Ce3+/Ce4+. (2) Labile oxygen vacancies can be easily formed in the CeO2 structure; thus the bulk oxygen species may have relatively high mobility, which facilitates the intercalation/ deintercalation of lithium ions.96,97 Galvanostatic tests were also applied to other CeO2 spherical crystallites with different Ce(NO3)3‚6H2O dosages and molar ratios of PVP (repeating units) to Ce(NO3)3‚6H2O. The results revealed that the initial discharge capacity of as-obtained CeO2 spherical crystallites mainly depended on their size. Table 1 reveals the connection between the size of CeO2 spherical crystallites and their initial discharge capacities. As the size of CeO2 spherical crystallites reduces from 600-800 nm to around 100 nm, the initial discharge capacity of the samples before calcination treatment increases accordingly from 400 to ∼680
mA h g-1, while the initial discharge capacity of the samples after calcination treatment increases from 480 to ∼820 mA h g-1. All the samples had excellent cycleability with more than 90% of the initial discharge capacity retained after 50 charge/ discharge cycles. The possible reason for this phenomenon could be that, along with the decrease of the crystallite size, the specific surface of as-obtained CeO2 spherical crystallites was accordingly enlarged, thus leading to more compositive CeO2 nanoparticles exposed to the electrolyte. As a result, the surface electrochemical reactivity between electrolyte and the electrode materials was greatly improved, which finally caused the improvement of the electrochemical properties. The above electrochemical test results revealed that as-obtained CeO2 spherical crystallites had excellent electrochemical properties for application as anode materials in lithium ion battery systems. Compared with other transitional oxides as anode materials, the initial discharge capacity of as-obtained CeO2 spherical crystallites could reach as high as 820 mA h g-1, which is higher than the discharge capacity of other tranisitonal metal oxides. The cycleability of the as-obtained CeO2 spherical crystallites was also very excellent. The excellent electrochemical properties of as-obtained CeO2 spherical crystallites indicated that they had promising electrochemical properties that might enable CeO2 to be applied as an anode material in Li ion batteries. Further research on the Li+ intercalation and deintercalation process into the CeO2 crystal structure is underway in our group to facilitate the application of CeO2 as an anode material as early as possible, which will be reported in a future report. 4. Conclusion In this paper, CeO2 spherical crystallites were successfully fabricated by a facile surfactant-assisted hydrothermal process via the oriented aggregation of small CeO2 nanoparticles in the diameter range of 5-10 nm. The participation of PVP played a vital role in the oriented aggregation process of CeO2 nanoparticles. Synthetic parameters such as Ce(NO3)3‚6H2O dosage and molar ratio of PVP (repeating units) to Ce(NO3)3‚ 6H2O could be deliberately regulated to control the size of asobtained CeO2 spherical crystallites in the range of 100-800 nm. Galvanostatic tests were applied to the as-obtained CeO2 spherical crystallites, indicating their promising features for application as an anode material in Li ion batteries. References and Notes (1) Service, R. F. Science 1996, 271, 920. (2) Sun, Y.; Mayers, G.; Xia, B. Y. N. Nano Lett. 2003, 3, 675. (3) Alivisatos, A. P. Science 1996, 271, 933. (4) Wang, F. D.; Dong, A. G.; Sun, J. W.; Tang, R.; Yu, H.; Buhro, W. E. Inorg. Chem. 2006, 45, 7511. (5) Wu, Y.; Cui, Y.; Huynh, L.; Barrelet, C. J.; Bell, D. C.; Lieber, C. M. Nano Lett. 2004, 4, 871. (6) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255. (7) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (8) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. D. Nature 2005, 437, 121. (9) Kato, K.; Yoshioka, T.; Okuwaki, A. Ind. Eng. Chem. Res. 2000, 39, 4148. (10) Kato, K.; Yoshioka, T.; Okuwaki, A. Ind. Eng. Chem. Res. 2000, 39, 943. (11) Nolan, M.; Watson, G. W. J. Phys. Chem. B 2006, 110, 2256. (12) Nolan, M.; Watson, G. W. J. Phys. Chem. B 2006, 110, 16600. (13) Nolan, M.; Parker, S. C.; Watson, G. W. Surf. Sci. 2006, 600 (14), 175. (14) Brosha, E. L.; Mukundan, R.; Brown, D. R.; Garzon, F. H.; Visser, J. H. Solid State Ionics 2002, 148, 61. (15) Tarnuzzer, R. W.; Colon, J.; Patil, S.; Seal, S. Nano Lett. 2005, 5, 2573. (16) Patil, S.; Kuiry, S. C.; Seal, S. Proc. R. Soc. B 2004, 460, 3569. (17) Stefanik, T. S; Tuller, H. L. J. Eur. Ceram. Soc. 2001, 21, 1967.
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