Ilmenite FeTiO3 Nanoflowers and Their Pseudocapacitance - The

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Ilmenite FeTiO3 Nanoflowers and Their Pseudocapacitance Tao Tao,†,‡ Alexey M. Glushenkov,*,† Hongwei Liu,§ Zongwen Liu,§ Xiujuan J. Dai,† Hua Chen,^ Simon P. Ringer,§ and Ying Chen† †

Institute for Technology Research and Innovation, Deakin University, Waurn Ponds, VIC 3217, Australia College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China § Australian Centre for Microscopy and Microanalysis, University of Sydney, NSW 2006, Australia ^ Centre for Advanced Microscopy, The Australian National University, Canberra, ACT 0200, Australia ‡

bS Supporting Information ABSTRACT: Pronounced and stable pseudocapacitance has been found in flowerlike FeTiO3 nanostructures that were synthesized from ball-milled ilmenite (natural mineral) under mild hydrothermal conditions. Each nanoflower is composed of many thin petals with a thickness of 5 20 nm and a width of 100 200 nm. The formation of these flowerlike nanostructures is attributed to a dissolution precipitation mechanism involving an intermediate sodium-containing phase. Electrochemical properties of the obtained FeTiO3 nanostructures are evaluated in aqueous electrolytes. The capacitance of 122 ( 14.5 F/g is measured in 1 M KOH aqueous electrolyte at the current rate of 500 mA/g, and 50 ( 6 F/g is retained at 5 A/g. The material has good long-term cycling stability. According to our data, FeTiO3 nanostructures show functionality as an electrode material for supercapacitors.

’ INTRODUCTION FeTiO3 (ilmenite) is a natural mineral with a well-known traditional application of being a feedstock for industrial production of TiO2.1 Ilmenite is available in large quantities (world’s total reserves are in excess of 680 million tons), cheap (with a price fluctuating around 100 USD per ton), and can be found in various geographical locations—America, Australia, Europe, Asia, and Africa.2 The same phase of FeTiO3 can be prepared in the laboratory, and nanostructured ilmenite may possess attractive properties, opening novel applications for FeTiO3. For example, heterojunctions of FeTiO3 nanodiscs and TiO2 nanoparticles have been shown to act as a photocatalyst with enhanced photocatalytic activity.3 Ilmenite FeTiO3 is a semiconductor with a band gap of about 2.54 2.58 eV4,5 and distinct magnetic properties,6 and investigation of relevant applications is worthwhile. In addition, as we show in this paper, it has promising electrochemical properties relevant to the area of energy storage. Nanostructured FeTiO3 is an interesting representative of functional oxides, and its potential requires to be explored carefully. Only a limited number of results have been reported on the successful synthesis of nanostructured ilmenite so far. Singlecrystalline FeTiO3 nanodiscs have been synthesized by a hydrothermal reaction at 220 C in aqueous tetrabutylammonium hydroxide from FeSO4 3 7H2O and titanium isopropoxide.3 Aggregates of FeTiO3 nanoparticles have been mechanochemically produced by Ohara et al. by milling of TiO2 in steel-milling equipment.7 Here, we propose a different method of obtaining nanostructured ilmenite FeTiO3 by downsizing bulk ilmenite. r 2011 American Chemical Society

The ilmenite gets ball-milled in the first step and is subsequently subjected to a mild hydrothermal treatment in aqueous NaOH solution. The nanostructured FeTiO3 forms in a special morphology of nanoflowers. Pseudocapacitance of FeTiO3 nanoflowers in aqueous solutions is demonstrated in the second part of the paper. This property is closely related to the application of materials in energy storage, and, more specifically, in electrodes of electrochemical supercapacitors. Supercapacitors can be described as a class of energy storage devices complementary to batteries since their main desired characteristic is not an ability to store the maximum possible amount of electric charge but, instead, an ability to absorb and release charge quickly, that is, high power density.8,9 Supercapacitors normally consist of two electrodes immersed into an electrolyte solution, and from the structural point of view, they resemble batteries much more than conventional capacitors.10 It is accepted now that electrodes of supercapacitors may operate via two types of typical mechanisms: the charge may be stored in an electric double layer formed on the electrode electrolyte interface by ions and electrons11 or by the so-called pseudocapacitive charge storage that involves fast surface redox reactions.12 Polymers and various transition-metal oxides, such as, for example, RuO2, MnO2, V2O5, Co3O4, and NiO,13 17 are under investigation for their prospective application in electrodes of supercapacitors due to the pseudocapacitance Received: April 10, 2011 Revised: July 28, 2011 Published: July 29, 2011 17297

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they possess. There is emerging interest to study ternary oxides incorporating two types of multivalent metal cations since such oxides may exhibit novel mechanisms of charge storage leading to higher capacitances and wide potential windows. FeTiO3 is an interesting example of a ternary oxide, and according to our results presented here, ilmenite FeTiO3 nanoflowers possess capacitance as high as 122 ( 14.5 F/g.

’ EXPERIMENTAL METHODS Preparation of FeTiO3 Nanostructures. Ilmenite powder (FeTiO3, supplied by Consolidated Rutile Ltd., Australia) was used as a starting material. Its chemical composition can be expressed as TiO2 (dry basis), 49.6%; iron (total), 35.1%; FeO, 32.8%; Fe2O3, 13.7%; Al2O3, 0.47%; Cr2O3, 0.25%; and SiO2, 0.45%. Ten grams of ilmenite powder and four hardened steel balls (diameter of 25.4 mm) were loaded into a stainless steel container of a magneto-ball mill (described in more detail elsewhere18). The mill was subsequently filled with Ar gas at a pressure of 100 kPa. Ball-milling was conducted for 150 h at room temperature. The magnet was located at the bottom of the mill at a 45 position in relation to the vertical direction, and the rotation speed was 160 rpm. Such a milling mode provides strong impact on the milled materials. FeTiO3 nanostructures were prepared from milled ilmenite by the following mild hydrothermal treatment. A 100 mL portion of 2 M NaOH aqueous solution was placed into a 250 mL Sch€ott bottle, and 1 g of the milled sample and a stirring rod were loaded into the solution. The Sch€ott bottle was immersed into a paraffin oil bath installed on top of a hot plate. The solution with the powder was heat-treated at 120 C in the presence of magnetic stirring (800 rpm) for 2 h. After the completion of each synthetic experiment, the suspension was filtered, and the obtained solid samples were washed and dried at 90 C for 4 h. Characterization. The samples were characterized by X-ray diffraction (XRD, Panalytical X’Pert PRO diffraction system, Cu X-ray source, λ = 1.5418 Å), scanning electron microscopy (SEM, Carl Zeiss Supra55vp instrument), and transmission electron microscopy (TEM, JEOL JEM-2200FS instrument working at 200 kV). Energy-dispersive X-ray spectrometry (EDS) was conducted using an Oxford X-max system installed within a Hitachi 4300 scanning electron microscope. The data were acquired at the accelerating voltage of 15 kV and working distance of 15 mm. The X-ray photoelectron spectroscopy (XPS) measurement was conducted using a K-alpha X-ray photoelectron spectrometer from Thermo Fischer Scientific with monochromatic X-rays focused to a 400 μm spot size. Excessive charging of the samples was minimized using a flood gun. Survey spectra were obtained at a pass energy of 100 eV while high-resolution peak scans were performed at a pass energy of 20 eV. Charging was corrected by assigning the lowest binding energy peak of C 1s to 285.0 eV. Electrochemical Measurements. A three-electrode cell was used for electrochemical measurements. The slurry for the working electrodes was prepared by mixing 70 wt % FeTiO3 nanostructures with 20 wt % carbon black (Aldrich no. 699633) and 10 wt % polyvinylidene difluoride (PVDF) in N-methyl-2pyrrolidinone (NMP). The slurry was subsequently spread onto pieces of titanium foil. Pt wire was used as a counter electrode, Ag/AgCl as a reference electrode in 3 M KCl and 1 M H2SO4, and Hg/HgO in 1 M KOH. Cyclic voltammetry (CV) and galvanostatic charge discharge were performed using a Solartron

Figure 1. SEM images of starting ilmenite (a) and the ilmenite after the hydrothermal treatment in 2 M NaOH aqueous solution at 120 C for 2 h (b). The insets show the morphology of particles at higher magnifications.

Figure 2. SEM images of the ball-milled sample (a, b) and the same sample after the hydrothermal treatment in 2 M NaOH solution for 2 h (c, d). A distinct morphology of nanoflowers is developed after the treatment.

1470E potentiostat/galvanostat. A detailed description of the concept of a three-electrode cell and the electrochemical techniques used (CV and galvanostatic charge discharge) can be found in appropriate textbooks, for example, ref 19.

’ RESULTS AND DISCUSSION Synthesis and Characterization of Nanoflowers. An SEM image of the original ilmenite powder, which consists of large particles with a typical size of 100 300 μm, is shown in Figure 1a. According to our results, the hydrothermal treatment of initial ilmenite (without preliminary high-energy ball-milling treatment) in 2 M NaOH aqueous solution at 120 C does not lead to noticeable changes in the material’s morphology. Ilmenite particles after such a treatment are shown in Figure 1b, and only minor polishing due to rubbing against each other is noticeable. There are no obvious differences in the morphology of the ilmenite samples before and after the exposure to 2 M NaOH solution, and the development of nanostructures is not observed in the treated sample. In contrast, it is apparent that the ball-milled ilmenite is involved in dramatic morphological changes when it is exposed to the alkaline solution under elevated temperature and pressure. The experimental conditions were kept the same. A survey SEM image of the initial morphology of the ball-milled sample is shown in Figure 2a, and a higher-magnification image showing detailed morphology is presented in Figure 2b. The milled sample consists of predominantly submicrometer agglomerates, 17298

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Figure 3. XRD patterns for the milled FeTiO3 and the resulting flowerlike nanostructures (strong lines from the JCPDS 01-075-1211 card are also shown at the bottom of the graph). Figure 5. Na 1s XPS spectra taken from the original ilmenite, the milled material, and the FeTiO3 nanoflowers.

Figure 4. TEM characterization of flowerlike FeTiO3: (a) A bright-field image showing a typical nanopetal, (b) the FFT pattern derived from (a), (c) an indexed pattern of FeTiO3 corresponding to the zone axis of [100], (d) an inverse FFT image showing (011) and (003) lattice fringes, and (e) top view of the (210) atomic plane.

and a closer inspection reveals that these agglomerates are composed of aggregated tiny particles with a typical size of several nanometers to tens of nanometers (Figure 2b). The morphology of the aggregates is rather typical for the products of dry high-energy ball milling,20 which commonly consists of nanocrystalline aggregates of small particles. The SEM image of the product after the treatment in the alkaline solution is shown in Figure 2c. This material is composed of uniform flowerlike architectures of about 1 2 μm in diameter. Each nanoflower consists of a number of petals with smooth surfaces (Figure 2d). Each petal is 5 20 nm thick and 100 200 nm wide, and different petals are interconnected. The quantitative EDS analysis reveals that the elemental composition (wt %) of nanoflowers is nearly identical to that of the original ilmenite FeTiO3. The XRD pattern of the flowerlike nanostructures (Figure 3) is very close to that of the milled ilmenite and agrees well with the standard XRD pattern of ilmenite FeTiO3 (JCPDS 01-0751211). The diffraction peaks of them are weak and broad. The broadening of the peaks is due to the small crystallite size. TEM

was further employed to investigate the crystallinity and orientation of petals. Figure 4 shows the results of the TEM characterization. A typical nanoplate (petal) of nanoflowers oriented in a convenient way (with its side wall being perpendicular to the electron beam) is depicted in Figure 4a. A fast Fourier transform (FFT) pattern derived from the plate shown in Figure 4a is presented in Figure 4b. It represents a periodic array of spots, which are slightly distorted into small arcs. Such a pattern indicates that the plate is either a single crystal with the presence of some defects (possibly dislocations) introducing minor rotations or can be described as a highly textured polycrystal in which nanocrystalline grains are slightly disoriented. The pattern can be indexed as the same phase of ilmenite FeTiO3, as shown by X-ray diffraction (Figure 3). The incident electron beam is close to the normal of the plate, which is, therefore, parallel to the [100] crystallographic direction (Figure 4c). The exposure plane of the nanopetal can be identified as (210). An inverse FFT image of the petal is shown in Figure 4d, and the characteristic angle between (011) and (003) planes has a good fit to the ideal atomic arrangement in the FeTiO3 crystal (Figure 4e). The nanoflowers were also characterized by XPS, and the results for Ti, Fe, and O elements, accompanied by a short discussion, can be found in the Supporting Information (section S1). It is only important to mention here that the data were consistent with the ilmenite FeTiO3 phase of the nanoflowers. A considerable amount of sodium was identified at the surface of nanoflowers. Figure 5 shows the Na 1s peak for the original ilmenite, milled material, and FeTiO3 nanoflowers. Whereas virtually no sodium can be detected in the original and milled ilmenites, a distinct Na peak appears in the spectrum of the flowerlike material. Approximately 10 at. % of sodium is estimated to be present at the surface of the nanoflowers. At the same time, our EDS analysis (which probes not only the very surface but also the bulk of the material) did not confirm a high Na content. We conclude that the Na-rich components are present predominantly on the surface of the material. These surface Nacontaining phases can be important intermediates in the growth mechanism, as we discuss in the following part of this paper. Similar morphologies of nanoflowers have been reported for some materials previously. The list includes iron alkoxide,21,22 β-Ni(OH)2,23 R-Ni(OH)2,24 BaMoO4,25 Fe3S4,26 PbTe,27 WS2,28 CaF2 and CaF2:Ln3+,29 and Cu3V2O7(OH)2 3 H2O.30 Most of these flowerlike morphologies are formed via a hydrothermal or solution based method, which suggests that similar 17299

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The Journal of Physical Chemistry C growth mechanisms are possible. Typically, “primary particles” form first from dissolved ingredients or products of a chemical reaction in the solution.21,31,32 These “primary particles” are normally aggregates of nanoparticles and grow further to form the morphology of nanoflowers. The initial step of formation of primary particles in our case happens not in the solution, but during preliminary ball-milling, which is capable of producing similar aggregates of nanoparticles. It is obvious that the growth of our FeTiO3 nanostructures strongly resembles the data of some other reports and, particularly, the growth of iron alkoxide nanoflowers described in ref 21. The Supporting Information contains the SEM analysis of the morphology of ilmenite nanostructures after 0.5, 1, and 2 h of hydrothermal treatment (Supporting Information, section S2). Ostwald ripening33,34 is considered as a probable mechanism for the secondary stage of growth by the majority of the authors. In such a process, small particles in the solution dissolve and the dissolved material reprecipitates on growing crystallites, leading to their gradual enlargement. Although we are not convinced that the Oswald ripening is a valid mechanism in our case (it was observed that the surface area of the milled ilmenite was increasing by almost 10 times from 2.7 to 26 m2/g in the course of the hydrothermal treatment), it is quite clear that the FeTiO3 nanoflowers form via a kind of dissolution precipitation process. It is unlikely that different types of crystal growth, such as bulk grain growth, bulk restructuring, or surface diffusion, may happen effectively enough at a temperature as low as 120 C. It is also apparent that nanoflowers do not form in pure water under similar conditions, and NaOH is required to be added to water in order to promote the formation of these nanostructures. We have treated the ball-milled ilmenite powder hydrothermally in pure water, and no visible changes were found in the morphology of the material. Ilmenite is not normally considered being capable of reacting with NaOH. In industrial procedures, for example, NaOH solutions are used to remove impurities (such as silica and some others) from ilmenite selectively while the phase of FeTiO3 itself stays intact. There is, however, some clear evidence that FeTiO3 can react with NaOH under hydrothermal conditions. For example, Marincovic et al.35 have converted ilmenite sand into NaxFexTi2 xO4, a CaFe2O4 structure type compound by reacting it hydrothermally with NaOH, and formation of sodium titanate Na2TiO3 is discussed as a result of reaction of ilmenite with NaOH and dissolved oxygen under hydrothermal conditions in another publication.36 We may, therefore, expect some reaction between ilmenite and sodium hydroxide in our experiments, too, and according to the fact that the presence of NaOH is critical for the formation of nanoflowers, this reaction is particularly important. Sodium titanate or another phase that forms as a result of chemical reaction between FeTiO3 and NaOH must be soluble in water in order for the dissolution precipitation mechanism to happen. A high concentration of sodium in the surface layers of ilmenite nanoflowers detected by XPS (Figure 5 and the corresponding paragraph) is an indirect evidence for the above scenario. As we have mentioned previously, there is a clear difference between the results of hydrothermal experiments with milled and original (unmilled) ilmenite, which is likely to originate from the differences in the structure and reactivities of these samples. First, ball-milling provides a special precursor of nanocrystalline aggregates of nanoparticles, which is, according to existing reports,23 32 important for the formation of nanoflowers via a solution-based

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Figure 6. Cyclic voltammetry curves of the FeTiO3 nanoflowers recorded at sweep rates of 5 and 50 mV/s in 3 M KCl (a), 1 M H2SO4 (b), and 1 M KOH (c) aqueous electrolytes.

process. Second, the milled FeTiO3 is expected to react with NaOH with higher reaction rates than those of unmilled FeTiO3. This is justified by a number of reports highlighting differences in reactivity and stability between milled and unmilled powders.37 40 For example, milled oxides of zinc and tin have been found to evaporate at lower temperatures than the unmilled oxide powders.37,38 The temperature of carbothermal reduction of ilmenite was significantly lowered from 1100 to 760 C after ball-milling.39 Chen et al. have reported the evidence of increased dissolution and low-temperature oxidation of ilmenite induced by high-energy ball-milling.40,41 We, therefore, expect that the milled ilmenite is much more reactive toward NaOH in the course of its hydrothermal treatment at 120 C. Electrochemical Properties. The properties of FeTiO3 nanoflowers were evaluated in several types of aqueous solutions. The motivation for these measurements is that a number of metal oxides have been previously found to have the so-called pseudocapacitive behavior. These metals are normally multivalent metals, and most of the systems tested are oxides involving a single type of metal atoms. It is of high interest to evaluate ternary oxides because it is expected that they may provide capacitances over larger potential windows and involve redox chemical reactions over two types of metal cations. Ilmenite FeTiO3 is, therefore, assessed here as one of such prospective ternary oxides. Figure 6 shows CV curves of the FeTiO3 nanoflowers in three aqueous electrolytes—3 M KCl, 1 M H2SO4, and 1 M KOH—at sweep rates of 5 and 50 mV/s, and pseudocapacitive behavior is 17300

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Figure 8. CV curves of FeTiO3 nanoflowers and the milled ilmenite measured in 3 M KCl aqueous electrolyte at 5 mV/s.

Figure 7. Galvanostatic charge discharge measurements: (a) charge discharge profiles measured at current rates of 0.5 and 2 A/g, (b) cyclic stability at 0.5 A/g, (c) capacitance retention at high current loads.

evident in all cases. The potential windows are 1.2 to 0.4 V vs Ag/AgCl in 3 M KCl, 0 0.9 V vs Ag/AgCl in 1 M H2SO4, and 1.2 to 0.4 vs Hg/HgO in 1 M KOH aqueous electrolyte. The CV curves taken at 5 mV/s are closer to the rectangular ones in 3 M KCl, whereas the CV in 1 M H2SO4 shows a pair of distinct redox peaks. The area enclosed by the CV curve is much smaller for the working electrode in 1 M H2SO4 electrolyte, suggesting that the charge storage ability of FeTiO3 in this electrolyte is lower than that in 3 M KCl or 1 M KOH. The CV curves taken at the sweep rate of 50 mV/s are elliptical in shape, which may be an evidence of limitations in conductivity of the electrode. Indeed, FeTiO3 is a semiconducting material, and effects of limited electrical conductivity are possible at high sweep or charging rates. Electrochemical properties of FeTiO3 nanoflowers were also assessed by galvanostatic charge discharge experiments. The testing was conducted in 1 M KOH electrolyte in the potential range of between 1.2 and 0.4 V vs Hg/HgO reference electrode at current densities ranging from 0.5 to 5 A/g. Figure 7a shows the galvanostatic charge discharge curves of the FeTiO3 nanoflowers at current rates of 0.5 and 2 A/g. The capacitance measured at 0.5 A/g is 122 ( 14.5 F/g. The long-term cycle stability was investigated at a constant current density of 0.5 A/g over 1000 cycles, and the graph of the capacitance as a function of the cycle number is shown in Figure 7b. No noticeable decay in the specific capacitance was found after 1000 cycles. Figure 7c shows the capacitance retention of the FeTiO3 nanostructures at different charge/discharge current densities (between 0.5 and 5 A/g). The FeTiO3 nanoflowers are able to retain the capacitance of 50 ( 6 F/g at a high current rate of 5 A/g .

We should note that galvanostatic charge discharge experiments conducted at slow current rates (50 and 100 mA/g) showed some deviation from the behavior shown in Figure 7. Particularly, the Coulombic efficiency deviates from being close to 100%, and charge or discharge branches of the profile may become extended. Figure 8 shows the comparison of CV curves of the FeTiO3 nanoflowers and the milled ilmenite in 3 M KCl aqueous electrolyte. It is obvious that the capacitance of nanoflowers is significantly higher since capacitances are proportional to the areas enclosed by the corresponding CV curves (given that the sweep rate is fixed and the amount of active material on the working electrode is similar). An improved electrochemical behavior of the flowerlike FeTiO3 nanostructures can be attributed to their high surface area (26 m2/g) and interconnected hierarchical structure of petals. The flowerlike morphology is expected to provide a larger electrode electrolyte contact area, good penetration of electrolyte throughout the electrode, and a convenient conduction path for electrons traveling in the active component of the electrode. Meanwhile, we cannot exclude that sodium-containing layers detected on the surface may also contribute to the differences since the shape of the CV curve for the milled ilmenite (a sodium-free material) is somewhat different from that of the FeTiO3 nanoflowers.

’ CONCLUSIONS We have demonstrated that flowerlike FeTiO3 nanostructures can be prepared via high-energy ball-milling of ilmenite and subsequent mild hydrothermal treatment in 1 M NaOH aqueous solution. The nanoflowers are composed of a number of petals with smooth surfaces, and each petal is 5 20 nm thick and 100 200 nm wide. A dissolution precipitation mechanism is proposed to explain the formation of the FeTiO3 nanostructures. It is demonstrated that the presence of NaOH in water is necessary for the formation of nanoflowers. It is speculated that an intermediate, soluble sodium-containing phase is formed via the reaction of FeTiO3 with NaOH, and a high sodium content on the surface is evidenced by XPS measurements. The pseudocapacitive properties of FeTiO3 nanostructures were demonstrated in a range of aqueous electrolytes: 1 M H2SO4, 3M KCl, and 1 M KOH. It is shown that the nanostructures show attractive values of capacitance in 3 M KCl and 1 M KOH electrolytes. More specifically, the capacitance of 122 ( 14.5 F/g was measured in 1 M KOH at the current rate of 0.5 A/g, and a reasonable fraction of this capacitance is retained at 5 A/g. The capacitance can remain virtually unchanged for 1000 cycles. The capacitance of ilmenite nanoflowers is 17301

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The Journal of Physical Chemistry C significantly enhanced with respect to that of the milled ilmenite, and such an enhancement is attributed to their high surface area (26 m2/g) and interconnected hierarchical structure of petals in nanoflowers. Sodium-containing layers on the surface may also contribute to the differences.

’ ASSOCIATED CONTENT

bS

Supporting Information. XPS characterization of FeTiO3 nanoflowers, SEM images of various stages of their growth, and analysis of measurements errors. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone: +61 3 52272931. Fax: +61 3 52271103.

’ ACKNOWLEDGMENT Financial support from the Australian Research Council under the Centre of Excellence program is acknowledged. T.T. thanks the National Basic Research Program of China (No. 2007CB613601) and the China Scholarship Council (CSC) for providing his scholarship. The authors also acknowledge the facilities and scientific and technical assistance from the Australian Microscopy and Microanalysis Research Facility (AMMRF) at the University of Sydney and from the XPS facility at RMIT University. ’ REFERENCES (1) Welham, N. J. Miner. Eng. 1996, 9, 1189–1200. (2) U.S. Department of the Interior, U.S. Geological Survey, Mineral 15 Commodity Summaries, 2009, http://minerals.usgs.gov/minerals/ pubs/commodity/titanium/mcs-2009-timin.pdf. (3) Kim, Y. J.; Gao, B. F.; Han, S. Y.; Jung, M. H.; Chakraborty, A. K.; Ko, T.; Lee, C. M.; Wan, I. L. J. Phys. Chem. C 2009, 113, 19179–19184. (4) Ginley, D. S.; Butter, M. A. J. Appl. Phys. 1997, 48, 2019–2021. (5) Zhou, F.; Kotru, S.; Pandey, R. K. Mater. Lett. 2003, 57, 2104–2109. (6) McDonald, P. F.; Parasiris, A.; Pandey, R. K.; Gries, B. L.; Kirk, W. P. J. Appl. Phys. 1991, 69, 110–1106. (7) Ohara, S.; Sato, K.; Tan, Z. Q.; Shimoda, H.; Ueda, M.; Fukui, T. J. Alloys Compd. 2010, 504, L17–L19. (8) Simon, P.; Gogotsi, Y. Nat. Mater. 2008, 7, 845–854. (9) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Kluwer Academic/Plenum Publishers: New York, 1999. (10) Winter, M.; Brodd, R. J. Chem. Rev. 2004, 104, 4245–4269. (11) Pandolfo, A. G.; Hollenkamp, A. F. J. Power Sources 2006, 157, 11–27. (12) Zhao, X.; Sanchez, B. M.; Dobson, P. J.; Grant, P. S. Nanoscale 2011, 3, 839–855. (13) Lee, C. Y.; Bond, A. M. Langmuir 2010, 26, 16155–16162. (14) Belanger, D.; Brousse, T.; Long, J. W. Electrochem. Soc. Interface 2008, 17, 49–52. (15) Lee, H. Y.; Goodenough, J. B. J. Solid State Chem. 1999, 148, 81–84. (16) Lu, W.; Liu, X. H.; Wang, X.; Yang, X. J.; Lu, L. D. Curr. Appl. Phys. 2010, 10, 1422–1426. (17) Srinivasan, V.; Weidner, J. W. J. Electrochem. Soc. 1997, 144, L210–L213. (18) Chen, Y.; Halstead, A. T.; Williams, J. S. Mater. Sci. Eng., A 1996, 206, 24–29.

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(19) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 2001. (20) Glushenkov, A. M.; Zhang, H. Z.; Chen, Y. Mater. Lett. 2008, 62, 4047–4049. (21) Zhong, L. S.; Hu, J. S.; Liang, H. P.; Cao, A. M.; Song, W. G.; Wan, L. J. Adv. Mater. 2006, 18, 2426–2431. (22) Li, X. Y.; Si, Z. J.; Lei, Y. Q.; Li, X. N.; Tang, J. K.; Song, S. Y.; Zhang, H. J. CrystEngComm 2011, 13, 642–648. (23) Sarkar, S.; Pradhan, M.; Sinha, A. K.; Basu, M.; Negishi, Y. C.; Pal, T. Inorg. Chem. 2010, 49, 8813–8827. (24) Ren, Y.; Gao, L. J. Am. Ceram. Soc. 2010, 93, 3560–3564. (25) Luo, Y. S.; Zhang, W. D.; Dai, X. J.; Y, Y.; Fu, S. Y. J. Phys. Chem. C 2009, 113, 4856–4861. (26) Cao, F.; Hu, W.; Zhou, L.; Shi, W. D.; Song, S. Y.; Lei, Y. Q.; Wang, S.; Zhang, H. J. Dalton Trans. 2009, 9246–9252. (27) Jin, R. C.; Chen, G.; Wang, Q.; Pei, J.; Sun, J. X.; Wang, Y. CrystEngComm 2011, 13, 2106–2113. (28) Li, X. L.; Ge, J. P.; Li, Y. D. Chem.—Eur. J. 2004, 10, 6163–6171. (29) Hou, S. Y.; Zou, Y. C.; Liu, X. C.; Yu, X. D.; Liu, B.; Sun, X. J.; Xing, Y. CrystEngComm 2011, 13, 835–840. (30) Sun, X. J.; Wang, J. W.; Xing, Y.; Zhao, Y.; Liu, X. C.; Liu, B.; Hou, S. Y. CrystEngComm 2011, 13, 367–370. (31) Park, J.; Privman, V.; Matijevic, E. J. Phys. Chem. B 2001, 105, 11630–11635. (32) Cheng, Y.; Wang, Y. S.; Zheng, Y. H.; Qin, Y. J. Phys. Chem. B 2005, 109, 11548–11551. (33) Voorhees, P. W. J. Stat. Phys. 1985, 38, 231–252. (34) Kabalnov, A. S.; Shchukin, E. D. Adv. Colloid Interface Sci. 1992, 38, 69–97. (35) Marinkovic, B. A.; Mancicb, L.; Jardim, M.; Milosevic, O.; Rizzo, F. Solid State Commun. 2008, 145, 346–350. (36) Amer, A. M. Hydrometallurgy 2002, 67, 125–133. (37) Glushenkov, A. M.; Zhang, H. Z.; Chen, Y. Mater. Lett. 2008, 62, 715–718. (38) Tao, T.; Glushenkov, A. M.; Chen, Q. Y.; Chen, Y. J. Nanosci. Nanotechnol. 2010, 10, 5015–5019. (39) Chen, Y.; Hwang, T. H.; Williams, J. S. Metall. Mater. Trans. A 1997, 28, 1115–1121. (40) Chen, Y. J. Alloys Compd. 1997, 257, 156–160. (41) Chen, Y.; Williams, J. S.; Campbell, S. J.; Wang, G. M. Mater. Sci. Eng., A 1999, 271, 485–490.

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