+ All Categories
Home > Documents > Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li...

Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li...

Date post: 17-Jul-2016
Category:
Upload: bhabani-sankar-swain
View: 215 times
Download: 0 times
Share this document with a friend
Description:
fsdf
10
This content has been downloaded from IOPscience. Please scroll down to see the full text. Download details: IP Address: 115.145.166.240 This content was downloaded on 10/12/2013 at 03:11 Please note that terms and conditions apply. Facile hydrothermal synthesis of porous TiO 2 nanowire electrodes with high-rate capability for Li ion batteries View the table of contents for this issue, or go to the journal homepage for more 2010 Nanotechnology 21 255706 (http://iopscience.iop.org/0957-4484/21/25/255706) Home Search Collections Journals About Contact us My IOPscience
Transcript
Page 1: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

This content has been downloaded from IOPscience. Please scroll down to see the full text.

Download details:

IP Address: 115.145.166.240

This content was downloaded on 10/12/2013 at 03:11

Please note that terms and conditions apply.

Facile hydrothermal synthesis of porous TiO2 nanowire electrodes with high-rate capability for

Li ion batteries

View the table of contents for this issue, or go to the journal homepage for more

2010 Nanotechnology 21 255706

(http://iopscience.iop.org/0957-4484/21/25/255706)

Home Search Collections Journals About Contact us My IOPscience

Page 2: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

IOP PUBLISHING NANOTECHNOLOGY

Nanotechnology 21 (2010) 255706 (9pp) doi:10.1088/0957-4484/21/25/255706

Facile hydrothermal synthesis of porousTiO2 nanowire electrodes with high-ratecapability for Li ion batteriesHyun-Woo Shim1,3, Duk Kyu Lee2,3, In-Sun Cho2, Kug Sun Hong2

and Dong-Wan Kim1,4

1 Department of Materials Science and Engineering, Ajou University, Suwon 443-749, Korea2 Department of Materials Science and Engineering, Seoul National University,Seoul 151-744, Korea

E-mail: [email protected]

Received 10 February 2010, in final form 21 April 2010Published 2 June 2010Online at stacks.iop.org/Nano/21/255706

AbstractAnatase TiO2 nanowires were successfully synthesized using a low-temperature hydrothermaltreatment on as-prepared one-dimensional (1D) hydrogen titanate nanowires (H2Ti3O7) at180 ◦C. The anatase TiO2 nanowires were porous in nature with a high specific surface area.These nanowires were characterized using transmission electron microscopy (TEM),high-resolution TEM, x-ray powder diffraction, Raman spectroscopy, andBrunauer–Emmett–Teller (BET) measurements. The topochemical phase transformationmechanism from H2Ti3O7 to anatase TiO2 is discussed. The porous anatase TiO2 nanowireelectrodes demonstrated an excellent cycling performance and superior rate capabilitiescompared with the H2Ti3O7 nanowires and the anatase TiO2 nanowires that were preparedthrough calcination at 700 ◦C. The porous anatase TiO2 nanowires exhibited a capacity of∼145 mA h g−1 at 1 C after 500 cycles and 115 mA h g−1 at 20 C. This improvement in thelong-term cycle stability and outstanding rate capability was explained by various microscopicobservations of the porous 1D nanostructured nature of the nanowires during the Liintercalation/deintercalation cycles.

S Online supplementary data available from stacks.iop.org/Nano/21/255706/mmedia

(Some figures in this article are in colour only in the electronic version)

1. Introduction

Titanium dioxide (TiO2) is a very widely studied materialbecause of its advanced application potential in many fieldssuch as photocatalytic activity for degrading organic pollutantsunder ultraviolet irradiation [1–3], catalyst supports [4, 5],gas sensors [6], electrochromic devices [7], and materials forenergy storage devices [8]. These functionalities of titaniumdioxide can be enhanced by expanding the surface area andcontrolling the size and shape via nanostructure formations.In recent years, nano-sized TiO2 materials, including one-dimensional (1D) nanostructured materials such as nanowires,

3 H W Shim and D K Lee contributed equally to this work.4 Author to whom any correspondence should be addressed.

nanorods, nanowhiskers, and nanotubes, have attracted a greatdeal of attention because of their interesting size-, shape-and dimensionality-dependent physicochemical properties andtheir potential applications in the fields of materials forconverting solar energy into electricity, electrochemicalcapacitors, supercapacitors, and electrode materials for high-power performance lithium ion batteries [9–20].

In particular, these 1D nanostructured TiO2 materialsare regarded as promising active lithium intercalation anodematerials with fast Li-intercalation/deintercalation becausethey provide shorter path lengths for both electronic andLi ionic transport, a higher electrode/electrolyte contactarea, and better accommodation of the strain of Li ionintercalation/deintercalation [21, 22]. Furthermore, recently,combined with these advantages, the porous materials

0957-4484/10/255706+09$30.00 © 2010 IOP Publishing Ltd Printed in the UK & the USA1

Page 3: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

Nanotechnology 21 (2010) 255706 H-W Shim et al

have received particular attention since they can be moreeffective in increasing the electrode stability and the Liintercalation capacity, especially at high charge/dischargerates, and therefore there is an increasing interest inthe development of nanostructured TiO2 as alternativeanode active materials with enhanced kinetics and high-ratecapabilities because the porosity plays an important role inthe intercalation/deintercalation reactions in energy storageapplications.

Recently, considerable effort has been focused onexploring diverse synthetic methods ranging from vapor-phasetechniques to solution-growth processes for the synthesisof 1D nanostructured TiO2. In particular, in the case ofnanobelts or nanotubes, it is well known that various typesof TiO2 nanostructures can be synthesized through strongalkali treatment under hydrothermal or non-hydrothermalconditions [23–26].

Among the several TiO2 polymorphs, anatase-type TiO2

nanostructures have long been studied and have been generallyconsidered the most electroactive Li intercalation host amongthese polymorphs, including the thermodynamically moststable rutile structure, the brookite structure, and TiO2-B,which is a metastable monoclinic modification of titaniumdioxide. These anatase TiO2 nanostructures can facilitatethe intercalation/deintercalation of Li ion charge/dischargebecause of their open crystal structure, which originates fromthe stacking of the zigzag units that consist of the highlydistorted edge-sharing TiO6 octahedra [27, 28]. Moreover,most of the previous works on lithium intercalation have shownthat only the anatase [29–31] and TiO2-B [11, 32] structuresprovide satisfactory results, although excellent electrochemicalperformance of the rutile structure was recently reported by Huet al [33].

Herein we report on the electrochemical performanceof pure anatase TiO2 nanowires that were synthesized usinga low-temperature hydrothermal treatment with hydrogentitanate nanowires as the precursor. The obtained anataseTiO2 nanowires have a high specific surface area, relatedto their porous structure. The electrochemical propertieswere evaluated and compared with the as-prepared hydrogentitanate nanowires and other anatase TiO2 nanowires that wereprepared by calcining hydrogen titanate at 700 ◦C. Our resultsclearly demonstrate that the porous anatase TiO2 nanowireswith a high specific surface area can offer enhanced storagereaction kinetics.

2. Experimental details

2.1. Synthesis

Hydrogen titanate nanowires (H2Ti3O7·nH2O) were synthe-sized using a hydrothermal treatment in an alkaline environ-ment. First, 0.1 g of commercial anatase powder (Sigma-Aldrich, USA) was dispersed in 20 ml of a 10 mol l−1 NaOHaqueous solution. The solution was stirred for 1 h at roomtemperature to ensure homogeneity, and then the suspensionwas transferred into a Teflon-lined stainless steel autoclave forthe hydrothermal treatment. The autoclave was maintained

at 200 ◦C for 48 h. After the hydrothermal treatment, thesuspension was centrifuged, and a white sodium titanate(Na2Ti3O7) precipitate was obtained. Then the precipitate waswashed and dried. The prepared precipitate was soaked in a0.1 mol l−1 HCl aqueous solution for 24 h at room temperature,which fully induced cation exchange between the sodium ionsand the hydrogen ions. Finally, hydrogen titanate nanowireswere obtained after the suspension was washed with deionizedwater several times.

The anatase TiO2 nanowires were fabricated from theas-prepared hydrogen titanate nanowires using two differentprocesses. First, in the conventional method, the hydrogentitanate nanowires were calcined at 700 ◦C for 8 h in air.The other method was a low-temperature synthesis through ahydrothermal treatment with deionized water at 180 ◦C for 3 halong with centrifugation and washing steps.

2.2. Characterization

The crystal structures and morphologies of the 1D TiO2

nanowires and the as-prepared hydrogen titanate nanowireswere investigated using x-ray powder diffraction (XRD;model M18XHF, Macsicence Instruments, Japan), Ramanspectroscopy (model T64000, Horiba Jovin Yvon, France),field emission scanning electron microscopy (FESEM; modelJSM-6330F, JEOL, Japan), and high-resolution transmissionelectron microscopy (HRTEM; model JEM-3000F, JEOL,Japan). Additionally, the specific surface areas and thepore-size distributions of the products were examined usingthe Brunauer–Emmett–Teller (BET; Belsorp-mini, BEL JapanInc.) method with a nitrogen adsorption/desorption process.

2.3. Electrochemical measurement

All of the electrochemical measurements, including the cyclicvoltammetry (CV) and galvanostatic charge/discharge tests,were carried out using half cells of two electrodes with lithiummetal as the counter electrode. For the measurements ofthe electrochemical properties of the samples, the workingelectrodes were prepared as pellets (∼1 cm diameter) bypunching and compressing a mixture that was cast onto a Cufoil. The mixture was composed of 60 wt% of the activematerials with 20 wt% Super P carbon black (MMM Carbon,Brussels, Belgium) and 20 wt% of a Kynar 2801 binder(PVDF-HFP) that was dissolved in 1-methyl-2-pyrrolidinone(NMP; Sigma-Aldrich). The cells were fabricated as lithiumbatteries in an Ar-filled glove box. The Swagelok-type cellswere assembled with the pellets as the positive electrode,Li metal foil as the negative electrode, and a Celgard 2400separator that was saturated with the electrolyte solution thatconsisted of 1 M LiPF6 dissolved in a mixture of ethylenecarbonate (EC) and dimethyl carbonate (DMC) with a volumeratio of EC/DMC = 1:1.

The electrochemical properties of the assembled cellswere recorded with the cyclic voltammetry curves and thecharge/discharge curves in a voltage window between 2.5and 1.0 V using an automatic battery cycler (WBCS 3000,WonATech, Korea). The cyclic voltammetry measurementswere carried out at a scanning rate of 0.1 mV s−1. Furthermore,

2

Page 4: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

Nanotechnology 21 (2010) 255706 H-W Shim et al

Figure 1. ((a), (c)) Typical FESEM images and ((b), (d)) magnified FESEM images of the HTO and HT–TiO2 nanowires, respectively.

the charge/discharge tests were galvanostatically cycledbetween 2.5 and 1.0 V for 500 cycles at a current rate of 1 C.These charge/discharge tests were also performed at variouscurrent rates of 1, 5, 10, and 20 C in order to confirm therate capability. All of the electrochemical measurements werecompleted at room temperature.

3. Results and discussion

The hydrogen titanate (H2Ti3O7·nH2O, hereafter HTO)nanowires as the precursor were prepared using previouslyreported hydrothermal procedures [34–36]. Additionally,we have also synthesized anatase TiO2 (hereafter HT–TiO2)nanowires by calcining the as-prepared the HTO nanowires ata high temperature of 700 ◦C, which has been reported as aconventional method for obtaining TiO2 structures of anatasephase [37].

Figures 1(a) and (c) show typical FESEM images of theHTO nanowires and the HT–TiO2 nanowires, respectively.The HTO nanowires had an average diameter of 200–400 nm(though some bundles comprising a few nanowires had adiameter of ∼1 µm) and length of several tens of micrometersand were successfully indexed to monoclinic H2Ti3O7 (figureS1 available at stacks.iop.org/Nano/21/255706/mmedia). Aftercalcination of the HTO nanowires at a temperature of morethan 700 ◦C, we observed complete conversion to TiO2 withthe anatase structure. The morphology of HT–TiO2 wasreminiscent of the HTO precursors even after the dehydration

reaction and further densification during the calcination at arelatively high temperature of 700 ◦C (figures 1(c) and (d)).

Not only the widely investigated anatase-type TiO2 butalso HTO have been recently reported as electroactive, Li-insertion hosts [38]. Figure 2(a) shows the galvanostaticcycling characteristics of the HTO nanowires using the HTOnanowires/Li half cell over a voltage window of 1.0–2.5 V at arate of 168 mA g−1. The first specific discharge capacity of theHTO nanowires reached a maximum value of ∼350 mA h g−1.The reversible capacity of these nanowires gradually fadedbut remained high, ∼100 mA h g−1, even after 500 cycles.Figure 2(b) presents the behavior of the voltage/capacity curvesfor the HT–TiO2 nanowires that were completely convertedfrom HTO to anatase TiO2 at 700 ◦C. The voltage profiles ofHTO nanowires show a sloped feature, indicating the natureof a single phase, lithium intercalation process, not a biphasicprocess. However, the behavior of HT–TiO2 nanowires wasunlike that of the HTO nanowires due to the different nature ofthe Li intercalation process [8, 38]. At a rate of 168 mA g−1

(1 C, based on the theoretical capacity of 168 mA h g−1 by thereaction, TiO2 + 0.5Li � Li0.5TiO2) [27], the capacity of HT–TiO2 drastically faded to a reversible capacity of 55 mA h g−1

after 500 cycles, which may have been attributable to therelatively small surface area and dense microstructure afterhigh-temperature annealing (figure 1(c)).

In order to obtain 1D TiO2 nanostructures with asufficiently large surface area, as-prepared HTO nanowireswere hydrothermally dehydrated at a low temperature of

3

Page 5: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

Nanotechnology 21 (2010) 255706 H-W Shim et al

Figure 2. Charging–discharging curves for (a) the HTO and(b) HT–TiO2 nanowire electrodes cycled between 2.5 and 1.0 V at arate of 168 mA g−1.

180 ◦C for 3 h. In figure 3(a), the x-ray diffraction (XRD)patterns for these powders indicated that they were pureanatase phase TiO2 (hereafter LT–TiO2) [39, 40]. The XRDpatterns of HTO and HT–TiO2 nanowires are also included forcomparison purposes. The structures of the LT–TiO2 and theas-prepared HTO nanowires were further verified using room-temperature Raman spectroscopy (figure 3(b)). The Ramanspectra of both the HTO and LT–, HT–TiO2 were consistentwith the spectra that were reported for the HTO and anataseTiO2 nanowires [36, 37, 41]. More importantly, the overall1D morphology of the starting HTO was clearly maintainedeven though the phase was completely transformed to anataseTiO2 after the hydrothermal treatment (figure 3(c)). Furtherinvestigation by transmission electron microscopy (TEM)(figures 3(d) and (e)) of LT–TiO2 revealed that the nanowireswere much rougher, slightly shrunken, and shorter. Moreimportantly, the LT–TiO2 nanowires exhibited a predominantlyporous structure because of the dehydration reaction that tookplace for the phase transformation during the hydrothermaltreatment. The presence of single phase anatase TiO2 was alsoconfirmed from the selective area electron diffraction (SAED)patterns in the inset of figure 3(e) and the typical HRTEMimage that was taken from a single nanowire with lattice

fringes of ∼0.352 nm, which corresponded to the interplanarspacing of anatase TiO2 (figure 3(f)). However, HT–TiO2

nanowires obtained after complete conversion to anatase TiO2

using calcination of the HTO nanowires at 700 ◦C had arelatively dense microstructure, although their morphologywas reminiscent of the HTO precursors (figures 3(g) and (h)).

The in situ phase transformation from HTO to anataseTiO2 while retaining the 1D morphology during low-temperature hydrothermal treatment and is closely relatedto the topochemical reaction due to the common structuralfeatures of HTO and anatase TiO2 [42]. Figure 4 shows thecrystal structures of HTO and anatase TiO2. In the HTOlattice, TiO6 octahedra share edges at one level in the linegroup, and each group interacts above and below with similargroups by further edge-sharing, resulting in a zigzag ribbonstructure, where the ribbons join at the terminal corners to formlayers, as shown in figure 4(a) [42–44]. Similar arrangementsand sharing of octahedra are also observed in the anataseTiO2 lattice (figure 4(b)). These similar structural frameworksmay favor the topochemical phase transformation leadingto the synthesis of porous anatase TiO2 nanowires throughdehydration during low-temperature hydrothermal treatment.

As described in microstructural observations in figure 3,the porous nature of the LT–TiO2 nanowires allows a largesurface area while retaining the 1D nanostructure. Both theHTO and HT–TiO2 nanowires had relatively small surfaceareas of 21 and 14 m2 g−1, respectively, which were measuredusing the BET method with nitrogen adsorption/desorption.However, it should be noted that a large specific areaof 95 m2 g−1 was obtained for the LT–TiO2 nanowires.The porous nature of the LT–TiO2 nanowires can bequantified with nitrogen adsorption/desorption measurementsand corresponding pore-size distribution as shown in figure S2(available at stacks.iop.org/Nano/21/255706/mmedia). Thehysteresis loop of isotherm and resultant pore sized distributionnear the 10 nm region are clearly observed. This can beunderstood by recognizing that such porosity is created by thedehydration process during phase transformation.

Nanostructured electrode materials with large surfaceareas have led to multiple advances in the performance of Liion batteries by providing shorter path lengths for both electronand Li ion transport, a higher electrode/electrolyte contactarea, and a better accommodation of the strain of the Li ioninsertion/extraction [21, 45]. The electrochemical performancewas evaluated for the porous 1D anatase TiO2 nanowireswith a large surface area that were successfully synthesizedvia the low-temperature hydrothermal route. Figure 5 showsthe electrochemical activity of all of the products that wereprepared in this work. These cyclic voltammetry (CV)measurements were carried out over five cycles using a halfcell with Li metal foil as the negative electrode, operating at0.1 mV s−1. The CV profiles show that the electrochemicalproperties of the samples are quite different. For the HTOnanowires (figure 5(a)), a pair of broad redox peaks appearedat ∼1.2 and 1.7 V during the first cycle, whereas the cathodicpeaks and anodic peaks were found for the following cyclesat ∼1.7 and 1.5 V. These results in good agreement with theresults that were obtained by Wei et al [38]. For the first cycle,

4

Page 6: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

Nanotechnology 21 (2010) 255706 H-W Shim et al

Figure 3. (a) XRD patterns and (b) Raman spectra of the HTO, LT–TiO2, and HT–TiO2 nanowires, respectively. Typical (c) FESEM and(d) TEM images of the LT–TiO2 nanowires. (e) Magnified TEM image of the LT–TiO2 nanowires with porous nanostructures. The inset showsthe corresponding SAED patterns. (f) HRTEM image of an individual LT–TiO2 nanowire. (g) TEM image of an individual HT–TiO2

nanowire. The inset shows its SAED patterns. (h) HRTEM image of an individual nanowire.

5

Page 7: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

Nanotechnology 21 (2010) 255706 H-W Shim et al

Figure 4. Schematic representation of the common crystal structuralfeatures of (a) H2Ti3O7 and (b) anatase TiO2.

the observed cathodic peak at ∼1.2 V is possibly related to theH2O decomposition reaction in the HTO nanowires.

This phenomenon was also evidenced in the highirreversibility capacities of the HTO nanowires during thefirst discharge process (see figure 2(a)). However, for theLT–TiO2 nanowires, a major pair of cathodic/anodic peakswas observed at ∼1.7 and ∼2.0 V, respectively, for theLi intercalation/deintercalation (figure 5(b)), which is inaccordance with the pair of peaks that were reported foranatase TiO2. Generally, these peaks are usually observedfor polycrystalline [46] and single crystal anatase [47] andit is well known that they correspond to the intercalationor deintercalation of Li. Two minor pairs of redox peaks

Figure 5. Cyclic-voltammograms of the (a) HTO, (b) LT–TiO2, and(c) HT–TiO2 nanowires at a scanning rate of 0.1 mV s−1.

(1.48 V/1.56 V and 1.54 V/1.64 V) are also observed infigure 5(b). These extra reactions are related to the other typeof surface storage mechanisms and capacitive effects, whichare strictly connected to the dimensions, the porosity, andthe surface areas of the materials [48, 49]. Our results weresimilar to those which have already reported for mesoporousTiO2 [50]. Although the other pairs of peaks associated withdiffering site occupations might be attributed to the formationof the discrete phase or imperfection of the TiO2 lattice, theyfacilitate the transport of Li through surface defects in thenanomaterials as well as the bulk materials [50, 51].

6

Page 8: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

Nanotechnology 21 (2010) 255706 H-W Shim et al

Figure 6 shows the typical charge/discharge curves foran electrode composed of the porous LT–TiO2 nanowiresthat were proposed in this study. In order to make themeasurements, the cell, which was composed of LT–TiO2

nanowires/Li half cell, was cycled at a current rate of 1 C(=168 mA g−1) with a potential window between 2.5 and1.0 V over 500 cycles. Distinct charge and discharge potentialplateaus were observed at around 1.70 and 2.0 V (versusLi+/Li), respectively. These plateaus were related to theredox peaks in the CV, indicating the typical electrochemicalcharacteristics of anatase TiO2. Remarkably, a high firstdischarge capacity of 268 mA h g−1 was observed as wellas a favorable cycling capability during the subsequentcharge/discharge steps after a significant drop in capacity inthe second cycle. Furthermore, the specific reversible capacitywas stable and exhibited a high value of ∼145 mA h g−1

even after 500 cycles. As described earlier, generally,the lithium intercalation/deintercalation mechanism in TiO2

is TiO2 + xLi+ + xe− = Lix TiO2. In that case, theinsertion coefficient, x , depends on the crystallography andmicrostructure of the materials, and is usually close to 0.5 inthe anatase phase. In this work, the insertion coefficient ismuch larger than the typical value for anatase TiO2 powders.A recent study demonstrated that porous polycrystalline rutileTiO2 can accommodate more Li (Li0.7TiO2) because of itshigh tolerance to volume changes [52–54]. Therefore, thehigh reversible capacity that was observed for the LT–TiO2

nanowires might have originated from the large specific surfacearea that resulted from the 1D porous nanostructure.

Additionally, the cells were first cycled at a rate of 1 C,and after every 10 cycles the rate was increased in stages to20 C in order to demonstrate the rate capability of the LT–TiO2 nanowires (figure 6(b)). As expected, the porous LT–TiO2

nanowires clearly exhibited a much better capacity at high ratesthan the corresponding capacities that were achieved underidentical conditions for the HTO and HT–TiO2 nanowires.Even at high-rate of 20 C, the LT–TiO2 nanowires exhibited aspecific capacity of 115 mA h g−1 as well as a high Coulombicefficiency of over ∼99% at each current rate, indicatingexcellent cycling stability and reversibility. These resultsindicate that the Li ion dynamics in electrode materials weregreatly influenced by the size, surface area and microstructures,such as the pore structures of the materials, which affectthe reaction with the lithium intercalation/deintercalation. Asmentioned earlier, the improved electrochemical performancemay be attributed to a shorter diffusion length for boththe electron and Li ion, and the larger electrode/electrolytecontract area of the LT–TiO2 nanowires compared with theHTO and HT–TiO2 nanowires, which facilitate Li intercalationand deintercalation.

The XRD analysis and TEM observations were conductedin the charged state in order to further elucidate theLi intercalation/deintercalation reaction for the LT–TiO2

nanowires. The LT–TiO2 nanowires/Li half cell weredisassembled after the cycles and dried in an argon-filledglove box in order to avoid exposure to air. Figure 7(a)shows typical XRD patterns of the fully delithiated LT–TiO2

nanowires after 10 cycles at a current rate of 1 C. It is

Figure 6. (a) Charging–discharging curves for the LT–TiO2 nanowireelectrodes that were cycled between 2.5 and 1.0 V at a rate of168 mA g−1 (1 C). (b) Rate capabilities for the HTO, LT–TiO2, andHT–TiO2 nanowire electrodes at different C rates.

clearly seen that the initial anatase structure was maintainedwithout any phase change. Additionally, the porous nanowirestended to sustain their original 1D structure at the 100thcharged state, as shown in figure 7(b). Surprisingly, theLT–TiO2 nanowires maintained their wire-like morphologieseven after 500 cycles, despite becoming slightly rougher andshorter. These observations indicate that the porous nanowirescan effectively cope with the mechanical strain that wasinduced by iteration of the Li intercalation/deintercalation,resulting in the stable cycle retention for the LT–TiO2 nanowireelectrodes. All of the spots in the SAED patterns inthe insets of figure 7(b) and (c) were indexed as anataseTiO2. The HRTEM image shows a lattice spacing of∼0.352 and 0.189 nm, assigned to the corresponding (101)and (200) planes of anatase TiO2, respectively, whichconfirms that the crystalline anatase structure was maintained(figure 7(d)).

Based on these detailed observations, it is consideredthat the porous 1D nanostructures of the LT–TiO2 nanowirescan effectively accommodate the mechanical strain, therebyenhancing the cycle stability and, especially, the rate capabilityof the anatase TiO2 electrodes.

7

Page 9: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

Nanotechnology 21 (2010) 255706 H-W Shim et al

Figure 7. (a) XRD patterns of the fully delithiated LT–TiO2 nanowires after 10 cycles at a rate of 1 C. The arrow indicates the peak thatoriginated from the copper current collector. Typical TEM images of the fully delithiated LT–TiO2 nanowires after (b) 100 and (c) 500 cyclesat a rate of 1 C. The insets in (b) and (c) show the corresponding SAED patterns. (d) HRTEM image of the fully delithiated LT–TiO2

nanowires after 500 cycles at a rate of 1 C.

4. Conclusions

In summary, we have demonstrated the synthesis of 1Dcrystalline anatase TiO2 nanowires with porous structures viaa hydrothermal treatment of hydrogen titanate nanowires at180 ◦C for 3 h. The obtained porous anatase TiO2 nanowireshad a high specific surface area of approximately 95 m2 g−1

and exhibited excellent electrochemical performance with ahigh specific capacity of 145 mA h g−1 even after 500 cycles at1 C. In line with previous reports on nanostructured TiO2, theporous anatase TiO2 nanowires studied here offer a superiorrate capability performance thanks to improved Li storagereaction kinetics induced by their porous 1D nanostructurednature. Therefore, these TiO2 nanowires may enable potentialhost materials for high-power Li ion batteries.

Acknowledgments

This work was supported by the Energy Efficiency andResources of the Korean Institute of Energy TechnologyEvaluation and Planning (KETEP) grant funded by theKorean Government Ministry of Knowledge Economy(No. 2008EEL11P080000).

References

[1] Linsebigler A L, Lu G Q and Yates J T 1995 Chem. Rev.95 735

[2] Fox M A and Dulay M T 1993 Chem. Rev. 93 341[3] Bhatkhande D S, Pangarkar V G and Beenackers A 2001 ACM

J. Chem. Technol. Biotechnol. 77 102[4] Chen M S and Goodman D W 2004 Science 306 252[5] Stiehl J D, Kim T S, McClure S M and Mullins C B 2004

J. Am. Chem. Soc. 126 1606[6] Fields L L and Zheng J P 2006 Appl. Phys. Lett. 88 263102[7] Hagfeldt A, Vlachopolous N, Gratzel M and Exnar I 1994

J. Electrochem. Soc. 141 L82[8] Lee D H, Park J G, Choi K J, Choi H J and Kim D W 2008 Eur.

J. Inorg. Chem. 878[9] Adachi M, Murata Y, Takao J, Jiu J T, Sakamoto M and

Wang F M 2004 J. Am. Chem. Soc. 126 14943[10] Armstrong A R, Armstrong G, Canales J and Bruce P G 2004

Angew. Chem. Int. Edn 43 2286[11] Armstrong A R, Armstrong G, Canales J, Garcia R and

Bruce P G 2005 Adv. Mater. 17 862[12] Gao X P, Zhu H Y, Pan G L, Ye S H, Lan Y, Wu F and

Song D Y 2004 J. Phys. Chem. B 108 2868[13] Hagfeldt A and Gratzel M 1995 Chem. Rev. 95 49[14] Joo J, Kwon S G, Yu T, Cho M, Lee J, Yoon J and

Hyeon T 2005 J. Phys. Chem. B 109 15297[15] Kavan L, Kalbac M, Zukalova M, Exnar I, Lorenzen V,

Nesper R and Graetzel M 2004 Chem. Mater. 16 477[16] Kim D W, Hwang I S, Kwon S J, Kang H Y, Park K S,

Choi Y J, Choi K J and Park J G 2007 Nano Lett. 7 3041[17] O’Regan B and Gratzel M 1991 Nature 353 737[18] Wu J J and Yu C C 2004 J. Phys. Chem. B 108 3377[19] Zhou Y K, Cao L, Zhang F B, He B L and Li H L 2003

J. Electrochem. Soc. 150 A1246[20] Zukalova M, Kalbac M, Kavan L, Exnar I and Graetzel M 2005

Chem. Mater. 17 1248

8

Page 10: Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes With High-rate Capability for Li Ion Batteries

Nanotechnology 21 (2010) 255706 H-W Shim et al

[21] Arico A S, Bruce P, Scrosati B, Tarascon J M andVan Schalkwijk W 2005 Nat. Mater. 4 366

[22] Kim D W, Ko Y D, Park J G and Kim B K 2007 Angew. Chem.Int. Edn 46 6654

[23] Chen Q, Zhou W Z, Du G H and Peng L M 2002 Adv. Mater.14 1208

[24] Kukovecz A, Hodos N, Horvath E, Radnoczi G, Konya Z andKiricsi I 2005 J. Phys. Chem. B 109 17781

[25] Patzke G R, Krumeich F and Nesper R 2002 Angew. Chem. Int.Edn 41 2446

[26] Tian Z R R, Voigt J A, Liu J, McKenzie B and Xu H F 2003J. Am. Chem. Soc. 125 12384

[27] Nuspl G, Yoshizawa K and Yamabe T 1997 J. Mater. Chem.7 2529

[28] Deng D, Kim M G, Lee J Y and Cho J 2009 Energy Environ.Sci. 2 818

[29] Zachau-Christiansen B, West K, Jacobsen T and Atlung S 1998Solid State Ion. 28–30 1176

[30] Huang S, Kavan L, Exnar I and Gratzel M 1995 J. Electrochem.Soc. 142 L142

[31] Kavan L, Gratzel M, Rathousky J and Zukal A 1996J. Electrochem. Soc. 143 394

[32] Armstrong G, Armstrong A R, Bruce P G, Reale P andScrosati B 2006 Adv. Mater. 18 2597

[33] Hu Y S, Kienle L, Guo Y G and Maier J 2006 Adv. Mater.18 1421

[34] Wang Y M et al 2008 Adv. Funct. Mater. 18 1131[35] Yu Y X and Xu D S 2007 Appl. Catal. B 73 166[36] Mao Y B and Wong S S 2006 J. Am. Chem. Soc. 128 8217[37] Wang Y F, Wu M Y and Zhang W F 2008 Electrochim. Acta

53 7863[38] Wei M D, Wei K W, Ichihara M and Zhou H S 2008

Electrochem. Commun. 10 1164

[39] Li G S, Li L P, Boerio-Goates J and Woodfield B F 2005 J. Am.Chem. Soc. 127 8659

[40] Zhang H, Finnegan M and Banfield J F 2001 Nano Lett. 1 81[41] Riss A, Elser M J, Bernardi J and Diwald O 2009 J. Am. Chem.

Soc. 131 6198[42] Zhu H Y, Gao X P, Lan Y, Song D Y, Xi Y X and Zhao J C

2004 J. Am. Chem. Soc. 126 8380[43] Burdett J K, Hughbanks T, Miller G J, Richardson J W and

Smith J V 1987 J. Am. Chem. Soc. 109 3639[44] Zhu H Y, Lan Y, Gao X P, Ringer S P, Zheng Z F,

Song D Y and Zhao J C 2005 J. Am. Chem. Soc. 127 6730[45] Lee D H, Kim D W and Park J G 2008 Cryst. Growth Des.

8 4506[46] Krol R V d, Goossens A and Meulenkamp E 1999

J. Electrochem. Soc. 146 3150[47] Hengerer R, Kavan L, Krtil P and Grazel M 2000

J. Electrochem. Soc. 147 1467[48] Sudant G, Baudrin E, Larcher D and Tarascon J M 2005

J. Mater. Chem. 15 1263[49] Wagemaker M, Borghols W J H and Mulder F M 2007 J. Am.

Chem. Soc. 129 4323[50] Kavan L, Rathousky J, Gratzel M, Shklover V and

Zukal A 2000 J. Phys. Chem. B 104 12012[51] Gover R K B, Tolchard J R, Tukamoto H, Murai T and

Irvine J T S 1999 J. Electrochem. Soc. 146 4348[52] Yue W, Randorn C, Attidekou P S, Su Z, Irvine J T S and

Zhou W 2009 Adv. Funct. Mater. 19 2826[53] Wang D, Choi D, Yang Z, Viswanathan V V, Nie Z, Wang C,

Song Y, Zhang J G and Liu J 2008 Chem. Mater. 20 3435[54] Dupont L, Laruelle S, Grugeon S, Dickinson C, Zhou W and

Tarascon J M 2008 J. Power Sources 175 502

9


Recommended