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Electrochemical properties of novel titania nanostructures

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2015 Nanotechnology 26 225603

(http://iopscience.iop.org/0957-4484/26/22/225603)

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Electrochemical properties of novel titaniananostructures

Sunqi Lou1, Fei Teng1, Juan Xu1, Zailun Liu1 and Yongfa Zhu2

1 Jiangsu Engineering and Technology Research Center of Environmental Cleaning Materials (ECM),Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), JiangsuJoint Laboratory of Atmospheric Pollution Control (APC), Collaborative Innovation Center of AtmosphericEnvironment and Equipment Technology (AEET), School of Environmental Science and Engineering,Nanjing University of Information Science and Technology, 219 Ningliu Road, Nanjing 210044, People’sRepublic of China2Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China

E-mail: tfwd@163.com

Received 13 January 2015, revised 1 April 2015Accepted for publication 14 April 2015Published 12 May 2015

AbstractIn this study, the supercapacitive properties of six new TiO2 nanostructures—includingnanodishes, three-layer nanosheets, ancient Chinese coins, single-layer nanosheets, hollownanocubes, and commercial rutile TiO2 are investigated mainly by cyclic voltammetry,chronopotentiometry, and electrochemical impedance spectroscopy. The results show thatamong them, the TiO2 nanodishes have the highest discharging capacitance at 1792 mFg−1,which is 6.4 and 1.5 times higher than that of TiO2 single-layer nanosheets and commercial rutileTiO2, respectively. We found that the electrochemical properties of the TiO2 samples arepredominated primarily by the high-energy facets exposed, instead of by the Brunauer–Emmett–Teller area. An important and previously unknown finding of our work is that theelectrochemical properties of electrode materials can be improved by controlling the high-energyfacets.

S Online supplementary data available from stacks.iop.org/NANO/26/225603/mmedia

Keywords: titania, nanostructures, supercapacitive

(Some figures may appear in colour only in the online journal)

1. Introduction

Today, energy shortages are becoming a grave problem forsocietal development. To solve this problem, we must exploresustainable and renewable energy sources. Supercapacitors,which are important energy storage technologies, haveattracted significant attention due to their high power density,long cycle life, and low maintenance cost [1–5]. Super-capacitors have been widely applied in electric vehicles,cellular phones, and camcorders [6–9]. From the viewpoint ofmaterials, supercapacitors can be classified as transitionmetals oxides [2, 5], carbonaceous materials [10], and con-ducting polymers [11]. From the viewpoint of energy-storagemechanisms, supercapacitors can be classified into pseudo-capacitance and electrical double-layer capacitance

categories. For transition metal oxide- and polymer-basedcapacitors, the energy storage mainly originates from Faradayredox reactions (i.e., pseudocapacitance) [12]. For carbonac-eous materials-based supercapacitors, the energy storage isbased on charge accumulation at the electrode/electrolyteinterface without any chemical reactions (i.e., electrical dou-ble-layer capacitances) [13]. Among various metal oxides,hydrous ruthenium oxide is an excellent supercapacitormaterial with a specific capacitance higher than 800 Fg−1 [14].However, its cost is too high to be widely used in practice.Thus, transitional metal oxides with abundant resources (e.g.,MnO2 [2, 15], TiO2 [16], etc) have attracted significantattention due to their low costs and high theoretical capaci-tances. However, their real capacitances are far from thetheoretical capacitances and their cycle performances are poor

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Nanotechnology 26 (2015) 225603 (9pp) doi:10.1088/0957-4484/26/22/225603

0957-4484/15/225603+09$33.00 © 2015 IOP Publishing Ltd Printed in the UK1

[17]. For polymers and carbonaceous materials, their appli-cations are limited by low capacitances [10, 18, 19]. Thus it isnecessary to search for a new, excellent electrode material tosolve these problems [20, 21].

In recent decades, titania (TiO2) has been investigatedextensively as a supercapacitor electrode material. Variousmethods have been used to improve its electrochemical per-formance, such as nanostructure control [22], array design[15, 16, 23], and the creation of composites with othermaterials [24]. However, the obtained results cannot meet thepractical requirements. Thus, much research effort is stillneeded to greatly improve the capacitive properties. Over thepast few years, anatase TiO2 nanosheets with their high-energy {001} facets exposed have been investigated inten-sively as photocatalysts, as they show high photocatalyticactivity [25, 26]. Herein, our attention focuses on whether theexposed high-energy facets of titania can affect the electro-chemical properties. If so, a new strategy can be developed toimprove the electrode material. Fortunately, our group hasdeveloped numerous new titania nanostructures, includingnanodishes, three-layer nanosheets, and ancient Chinese coins[27, 28], which allowed us to systematically explore theirfacet-dependent electrochemical properties. Interestingly, wefound that the electrochemical properties of titania are mainlypredominated by the high-energy surfaces, instead of by theBrunauer–Emmett–Teller (BET) area. In previous studies, theBET area usually had great influence on the electrochemicalproperties. This study may provide us with a new concept thatthe electrochemical properties of electrode materials can beefficiently improved by surface control.

In this study, the electrochemical properties of titaniananostructures are mainly investigated by cyclic voltammetry(CV), chronopotentiometry (CP), and electrochemical impedancespectroscopy (EIS). The influence of high-energy facets on theelectrochemical properties of the electrode has been revealed.

2. Experimental

2.1. Sample preparation

Typically, 1.95 g of water, 0.54 g of 40 wt% HF, and 30.03 gof 99.5 wt% HAc were mixed to form a clear solution, whichwas then mixed with 5.00mmol of tetrabutyl titanate in aTeflon-lined stainless steel autoclave with a capacity of 50mL.The autoclave was heated and then kept at 180 °C for 24 h.After the autoclave was cooled to room temperature naturally,the sample was separated by centrifugation, washed severaltimes with distilled water, and dried at 60 °C for 12 h. Hereinthe as-prepared sample is designated as Sample 1 (nanodishes).

For the TiO2 three-layer nanosheets (Sample 2) andancient Chinese coins (Sample 3), the same procedures wereused as for Sample 1, but the hydrothermal times wereincreased to 36 h and 48 h, respectively. For the TiO2 single-layer nanosheets (Sample 4), the same procedures were usedas for Sample 1, but a small amount of HF (0.375 g of 40 wt%HF) was used. For the preparation of TiOF2 nanocubes(Sample 5), we followed the same procedures as for Sample

1, but a large amount of HF (0.75 g of 40 wt% HF) was used.For the preparation of hollow TiO2 nanocubes (Sample 6), theas-synthesized Sample 5 above was annealed at 600 °C for 1 hin a muffle oven. To compare with the as-prepared samples,the commercial rutile TiO2 nanoparticles were also investi-gated, which is designated as Sample 7.

2.2. Electrode fabrication and electrochemical test

The working electrodes were fabricated by mixing 80 wt%TiO2, 10 wt% acetylene black, 10 wt% poly(vinylidenefluoride). The mixture was dispersed in 1-Methyl-2-pyrroli-dinone to form a homogeneous slurry under stirring. Theslurry was dotted on the surface of a nickel foam electrodeand then dried for 24 h at room temperature.

All the electrochemical measurements were carried outon an electrochemical working station (CHI 660D). Electro-chemical measurements were carried out in a three-electrode-type cell at room temperature. In the test, a platinum wireserved as a counter electrode, a saturated calomel electrode(SCE, Hg/Hg2Cl2) electrode was employed as a referenceelectrode, and 1M KNO3 aqueous solution was used as theelectrolyte. CV and CP were conducted in a potential range of0.1–0.9 V (versus SCE). EIS was performed from 0.1 Hz to100 KHz at an open circuit potential of 0.8 V and an alter-nating current (AC) voltage amplitude of 5 mV. The datawere analyzed by ZSimWin software.

2.3. Characterization of the material

The morphologies of the samples were characterized byscanning electron microscopy (SEM, Hitachi, SU-1510) usingan accelerating voltage of 15 kV. The fine surface structuresof the samples were characterized by high-resolution trans-mission electron microscopy (HRTEM, JEOL JEM-2100F)equipped with an electron diffraction attachment with anacceleration voltage of 200 kV. The crystal phases of thesamples were characterized by x-ray diffraction (XRD,Rigaku D/max-2550VB) with graphite monochromatized CuKα radiation (λ= 0.154 nm). The surface areas of the sampleswere measured by nitrogen sorption isotherms at 77 K on aNOVOE 4000 adsorption apparatus. The surface areas of thesamples were calculated by the BET method.

3. Results and discussion

3.1. Crystal phases and structures of the samples

The crystal phase of the resultant sample is confirmed byXRD (figure 1). All the diffraction peaks of Samples 1–4 and6 (i.e., nanodishes, three-layer nanosheets, ancient Chinesecoins, single-layer nanosheets, and hollow nanocubes) can bewell indexed to the phase-pure anatase TiO2 (referring toJCPDS no. 21-1272). Samples 5 and 7 can be well indexed tothe phase-pure TiOF2 (JCPDS no. 08-0060) and rutile TiO2

(JCPDS no. 21-1276), respectively. Moreover, we calculatedthe intensity ratios of (101)/(004) peaks (Samples 1–4 and 6),which are markedly higher than that of bulk anatase titania

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(figure 2(b)). This suggests that the {101} facets of thesamples grow preferentially. In particular, the {101} facets ofSample 1 grow more significantly.

Furthermore, the samples are characterized by SEM infigure 3. Figure 3(a) shows that the TiO2 nanodish form, whosecenter region is etched clearly but is not hollow (Sample 1).Further, the geometric structure is shown in the inset offigure 3(a). In figure 3(b), the TiO2 three-layer nanosheet form(Sample 2) is composed of two small nanosheets measuring2 μm×2 μm closely attached or stuck on the both sides of onelarge nanosheet measuring 5.5 μm×5.5 μm. Figure 3(c) showsthat the center regions of the TiO2 nanosheets became hollow,similar to the ancient Chinese coins (Sample 3). Figure 3(d)shows that the single-layer nanosheets of 600–900 nm haveformed (Sample 4). Figure 3(e) shows that the uniform TiOF2nanocubes have formed, at a size of ca. 500–800 nm (Sample5). Figure S1 shows that the hollow TiO2 nanocubes (Sample6) are ca. 500–800 nm in size (see the Electronic SupportingInformation). The commercial rutile TiO2 (Sample 7) have theaverage sizes of 700 nm (figure 3(f)). Moreover, the size uni-formities of all the samples are provided in figure S2 (see theElectronic Supporting Information).

3.2. Supercapacitive properties of the samples

The CV measurements of the samples are performed at ascanning rate of 10 mV s−1 in 1M KNO3 aqueous solution(figure 4(a)). The normative rectangular CV curves denote thetypical electrical double-layer capacitive behavior, instead ofthe Faraday reaction. The specific capacitance (C) can becalculated using equation (1) as follows.

Δ= ×C Q m V/ ( ) (1)

where C is the specific capacitance calculated based on themass of active material (Fg−1), Q is the charge calculated

using half the integral area of the CV curve, m is the mass (g)of the active material, and ΔV is the potential window. The Cvalues of the samples are summarized in table 1. Thecapacitance values of Samples 1, 3, and 5 are 1792, 136, 1and 1493 mFg−1, respectively. These values are much higherthan those of the other samples. In addition, the capacitance ofSample 7 is 1193 mFg−1. It is surprising that the capacitancevalue of Sample 1 is 1.5 times higher than that of commercialrutile titania. Also, the capacitance of Sample 3 is1361 mFg−1, which is 1.14 times higher than that ofcommercial rutile titania. Summarily, Sample 1 shows thelargest capacitance of all the samples.

Figure 4(b) shows the galvanostatic charge-dischargecurves of the samples at a current density of 1.0 mAcm−2. Thecharge/discharge curves indicate a typical electric double-layer capacitive behavior, which is in agreement with theresults of the CVs. The discharging capacitance can be cal-culated according to equation (2) as follows.

Δ Δ= × ×C I t m V/ ( ) (2)

Figure 1. XRD patterns of the samples. (a) Sample 1 (TiO2

nanodishes). (b) Sample 2 (TiO2 three-layer nanosheets). (c) Sample3 (TiO2 ancient Chinese coins). (d) Sample 4 (TiO2 single-layernanosheets). (e) Sample 5 (TiOF2 nanocubes). (f) Sample 6 (hollowTiO2 nanocubes). (g) Sample 7 (commercial rutile TiO2).

Figure 2. (a) XRD patterns of the samples and (b) the intensity ratiosof {101}/{004} peaks of Sample (1, 2, 3, 4, 6) and the standardanatase TiO2.

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where I represents the charging current density (Acm−2), Δt isthe discharging time (s), and ΔV is the discharging potential(V). At a current density of 1.0 mAcm−2, the dischargingcapacitances of Samples 1 and 5 are calculated to be 1729 and1361 mFg−1, respectively. These results are good in agree-ment with those of the CV measurements.

Generally, EIS is used both to investigate the electricalconductivity and ion transfer of supercapacitor test cells and tostudy the electrochemical processes with different time con-stants. Figure 5(a) shows the Nyquist plots of the electrodes ina frequency range from 0.1 Hz to 100 kHz, operated at an opencircuit potential of 0.20 V using an ac voltage amplitude of5 mV. Furthermore, figure 6 presents the simulated equivalentelectric circuit, which is well fitted to the EIS data. Obviously,all the EIS spectra have semicircles in the high-frequency

region, followed by slopes in the low-frequency region. Theintercept of the real part (Z′) at the beginning of the semicirclerepresents the internal resistance (Rs) (figure 5(b)). The semi-circle is related to the charge transfer resistance (Rctr), whichcan be evaluated by the diameter of the semicircle. The slope ofthe straight line represents the Warburg impendence (W),which is caused by the ion diffusion from bulk solution toelectrode surface. In the low-frequency region, the larger theslope of the straight line, the smaller the diffusion impedance.The Rs values of the electrodes are the same, meaning thatseven electrodes have the same intrinsic resistance becausethey have the same contact resistance at the interface of theactive material/current collector [29, 30]. The electrode fabri-cated by Sample 1 has a smaller Rctr than Sample 7, as indi-cated by its smaller semicircle (figure 5(a)), but its semicircle is

Figure 3. SEM images of the samples. (a) Sample 1 (the inset of geometric diagram); (b) Sample 2; (c) Sample 3; (d) Sample 4; (e) Sample 5;(f) Sample 7.

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larger than those of Samples 3 and 5 (figure 5(b)). However,Sample 1 has the largest capacitance among the four electro-des. This demonstrates a higher conductivity for the electrodeby Sample 1, probably resulting from the nanodish morphol-ogy and the high-energy surface exposed [30]. The nanodishstructure may endow Sample 1 with a large BET area, and thehigh-energy surface may endow it with a fast charging rate.This will be discussed later in the paper. Moreover, the elec-trodes fabricated by Samples 3 and 5 have smaller Rctr than theelectrode fabricated by Sample 1, but in the low-frequencyregion, their slopes of the straight lines are slightly smaller thanthat found in Sample 1 (figure 5(b)). The slope of the straightline of Sample 1 is slightly steeper than those of the others,indicating that the former electrode has a smaller diffusionimpedance and a faster formation speed of the electric double-layer [31]. It is well known that both Rctr and the diffusionimpedance have significant influence on conductivity, thusimpacting the material’s capacitance. In figure 5(a), Samples 1,3, and 5 have the nearly same Rctr values, but their diffusionimpedances are obviously different. The diffusion impedancewould affect the inductive impedance and stability of theelectrode, thus affecting the measurement result. Hence,

Sample 1 has a small diffusion impedance and high stability,making it an excellent supercapacitor material.

3.3. Effect of high-energy surface exposed

It is well known that the electrochemical properties of anelectrode can be affected by the physicochemical properties ofa material, including the BET area, electric conductivity,crystal structure, nanostructure, etc [1, 16]. Usually, the tex-tural properties and electric conductivity of a material havegreat influence on the capacitive properties of the electrodes

Figure 4. (a) Cycle voltammograms of the samples at a scan rate of10 mV s−1. (b) The charge–discharge curves of the samples at acurrent density of 1.0 mA cm−2 in 1 M KNO3 aqueous solution.

Figure 5. Nyquist plots of the electrodes in the frequency range from0.1 Hz to 100 kHz at an open circuit potential of 0.20 V in 1 MKNO3 aqueous solution. (a) Nyquist plots in the whole frequencyrange. (b) Nyquist plots in the high frequency range of Samples 1–6.

Figure 6. Simulated equivalent electric circuit of the impedancespectra. 1: electrolyte resistance. 2: sample/electrolyte interfacecapacitance. 3: Warburg impedance. 4: sample impedance. 5: samplecapacitance.

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Figure 7. HRTEM images (the insets of SAED patterns) of the samples. (a)–(c) Sample 1. (d)–(f) Sample 2. (g) and (h) Sample 3. (i) and (j)Sample 5. (k) and (l) Sample 6.

Table 1. The BET areas and specific capacitance values of the samples.

Samples Specific capacitance (mF g−1) BET areasa (m2g−1) Featured structureb

Sample 1 1729 68.5 TiO2 nanodishesSample 2 460 47.1 TiO2 three-layer nanosheetsSample 3 1361 51.6 TiO2 ancient Chinese coinsSample 4 270 12.6 TiO2 single-layer nanosheetsSample 5 1493 4.6 TiOF2 nanocubesSample 6 365 0.9 Hollow TiO2 nanocubesSample 7 1193 50.0 Commercial rutile TiO2

a

BET areas calculated by the Brunauer–Emmett–Teller method.b

Characterized by XRD, HRTEM, and SEM.

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[32]. The large BET area favors the accommodation of morecharges and increases the charge/discharge rate. In table 1, theBET areas of Samples 1–7 are measured as 68.5, 47.1, 51.6,12.6, 4.6, 0.9, and 50.0 m2g−1, respectively. Sample 1 has thelargest BET area, which could be ascribed to its rough surfacein the center region. The discharging capacitance of Samples1 and 3 are 1792 and 1361 mFg−1, which are 1.50 and 1.14times as high as that (1193 mFg−1) of Sample 7, respectively.However, Samples 3 and 7 have nearly the same surfaceareas, indicating that the crystal phase has a significantinfluence on the capacitive properties. Note that the BET area(0.9 m2g−1) of Sample 6 is far smaller than that (68.5 m2g−1)of Sample 1, but its capacitance reaches 20.4% of thatof Sample 1. Hence, we hold that in addition to the BETarea, there should be other factors that predominate thecapacitive properties because of their significantly differentcapacitances.

Herein, we have mainly investigated the surface struc-tures of the samples. HRTEM and selected-area electrondiffraction (SAED) are used to analyze the exposed surfacesof the samples. Figures 7(a)–(c) also confirm the formation ofTiO2 nanodishes (Sample 1), which is in agreement with theSEM result. The lattice fringe spacing of Sample 1 is deter-mined to be 0.35 nm, corresponding to the (101) plane ofanatase TiO2, indicating that the nanosheets grow pre-ferentially along the [101] direction [33]. As shown in theSAED patterns in the insets of figures 7(b) and (c), the per-pendicular lattice spacings of 0.35 nm represent the (200) and(010) planes of anatase TiO2.The set of diffraction spots canbe indexed as the [001] zone axis of anatase TiO2, indicatingthat the {001} facets are indeed mainly exposed. CombingXRD, HRTEM, and SAED analyses results, the crystal-lographic {001} facets are mainly exposed and parallel to thesurface of the nanoplates. As demonstrated by many studies

Figure 7. (Continued.)

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[25, 26, 33–36], the top and bottom surfaces of the TiO2

nanodishes are composed of the {001} facets, and the sidesurfaces are composed of the {101} facets. Identically, thesurfaces of Sample 3 with a hollow central region are alsocomposed of the {001} facets (figures 7(g) and (h)). Theexposure percentages of the {001} facets are calculated to be76.5% for both Samples 1 and 3. The theoretical calculationresults [26, 33] have demonstrated that for anatase TiO2, thesurface energies of typical facets are ordered as follows:{001} (0.90 J M2) > {100} (0.53 J M2) > {101} (0.44 JM2). Itis clear that the high-energy {001} facets are mainly exposedfor Samples 1 and 3. In contrast, there is no facet orientationfor commercial rutile TiO2 (Sample 7) (i.e., its surfaces arecomposed of thermodynamically stable facets). The resultsindicate that the high-energy surface of the materials has asignificant influence on the capacitive properties. Figure 7(i)also confirms the formation of TiOF2 nanocubes of approxi-mately 400 nm in size (Sample 5). In figure 7(j), the latticefringe spacing is determined to be 0.37 nm, corresponding tothe {100} facets of TiOF2 [34]. It has been reported thatfluorine ions can greatly reduce the surface energy of {001}facets and thus greatly restrict the growth of {001} facets,leading to a percentage of {001} facets exposed [2, 6, 35].Moreover, it has been reported that the O in TiO2 can besubstituted by F, and then Ti-F bonds can be formed [35]. It is

reasonable that both the substitution and the surface adsorp-tion of fluorine ions occur simultaneously. In the former case,the substitution of fluorine for the oxygen site needs one extraelectron for charge compensation, which would drive thenegative charge transfer to the electrode from the electrolytesolution [36, 37]. In the latter case, the adsorption of fluorineon the surface can also attract more negative charges from theelectrolyte solution due to its high electronegativity. As aresult, the TiOF2 sample has a high capacitance(1493 mFg−1).

Figure 7(k) further shows the hollow structure of Sample6, and the lattice fringe spacing is determined to be 0.19 nm(figure 7(l)). The percentage of the {001} facets exposedaccounts for 83% [26, 35, 36], and its discharging capacitance(365 mFg−1) is 20.4% of that (1729 mFg−1) of Sample 7.However, its BET area (0.9 m2g−1) is far smaller than that(68.5 m2g−1) of Sample 7. The result also indicates that thehigh-energy surface, instead of the BET area, plays the pre-dominant role in improving the capacitance. Moreover, the{001} facets are mainly exposed for Samples 1 and 2(figures 7(d)–(f)). Although the BET area (68.5 m2g−1) ofSample 1 is merely 0.45 times higher than that (47.1 m2g−1)of Sample 2, the capacitance (1729 mFg−1) of Sample 1 is3.76 times higher than that (460 mFg−1) of Sample 2. Thefairly low capacitances may be closely related to its three-layer structure. During electrochemical reaction, the attachednanosheets may be not stable and may easily break intopieces, resulting in a small capacitance.

Summarily, we could assume that the high-energy facetsfavor the adsorption and accumulation of charges, whichwould increase the electrochemical capacitance. It is wellknown that the titania surfaces are comprised of Ti and Oatoms, and the O–O and Ti–O bonds tend to become flat.Yang et al reported that in the {001} facets, the balancebetween the O–O repulsion and the attractive Ti–O π inter-action can be broken, resulting in the cleavage of surfaces.This would cause the unsaturated O and Ti atoms to moveoutward [26], enlarging the contact area of Ti with the elec-trolyte solution. Furthermore, the high-energy {001} facetshave a tendency to reduce the surface energy, thus becomestable thermodynamically. Therefore, the high-energy facetwould favor for the accommodation of more charges andwould increase the charging rate. As a result, the highercharging rate leads to higher capacitance at the same potentialwindow and the same scanning rate

3.4. The rate capacity and cycle stability

Figure 8(a) shows the rate capacity of Sample 1. At currentdensities of 1.0, 2.0, and 4.0 mAcm−2, the discharging capa-citances are 1729, 865, and 628 mFg−1, respectively. Thedischarging capacitance decreases with an increase in currentdensity, which could be ascribed to the different utilizationsof active surfaces at different current densities. At a lowcurrent density, the active surface of the electrode can be fullyavailable because there is enough time for charge accumula-tion at the electrode/electrolyte interface. At a high currentdensity, on the contrary, only the low-percentage active

Figure 8. (a) Rate capability and (b) cycle stability for 1000 cycles ofSample 1 in 1 M KNO3 aqueous solution.

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surfaces can be available. When the current density increasesfrom 1.0 to 4.0 mAcm−2, 36% of capacitance is still retained,demonstrating a high rate capability of the sample. Further-more, we have investigated its cycle stability (figure 8(b)).After 1000 cycles at 0.1 mAcm−2, 99% discharging capaci-tance is maintained. The columbic efficiency (η) is calculatedaccording to equation (3) as follows.

η = t t/ (3)1 2

where t1 and t2 represent discharging and charging times,respectively. After 1000 cycles, the columbic efficiencyremains 98% for the sample. The excellent stability can beattributed to the electrical double-layer capacitive mechanism,instead of to Faraday reactions.

4. Conclusions

The TiO2 nanodishes show high capacitance and excellentcycle stability, which are mainly attributed to the high-energy{001} facets exposed, instead of to the BET area. It isimportant that the electrochemical properties of super-capacitor materials can be effectively improved by controllingthe high-energy facets exposed, which was not previouslyknown. This could provide us with new ideas on using sur-face control to improve the electrochemical properties ofsupercapacitor materials.

Acknowledgments

This work is financially supported by National ScienceFoundation of China (21377060, 21103049), the ProjectFunded by the Science and Technology Infrastructure Pro-gram of Jiangsu (BM201380277, 2013139), Jiangsu ScienceFoundation of China (BK2012862), Six Talent ClimaxFoundation of Jiangsu (20100292), Jiangsu Province ofAcademic Scientific Research Industrialization Projects(JHB2012-10, JH10-17), the Key Project of EnvironmentalProtection Program of Jiangsu (2013016, 2012028), A ProjectFunded by the Priority Academic Program Development ofJiangsu Higher Education Institutions (PAPD) sponsored bySRF for ROCS, SEM (2013S002), and ‘333’ OutstandingYouth Scientist Foundation of Jiangsu (2011015).

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