Composites of rutile TiO2 nanorods loaded on graphene oxide nanosheet with enhanced electrochemical performance

Accepted Manuscript Title: Composites of rutile TiO2 nanorods loaded on graphene oxide nanosheet with enhanced electrochemical performance Author: Ruirui Liu Wenjun Guo Bin Sun Jinli Pang Meishan Pei Guowei Zhou PII: DOI: Reference:

S0013-4686(15)00015-8 http://dx.doi.org/doi:10.1016/j.electacta.2015.01.012 EA 24060

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

27-8-2014 20-11-2014 3-1-2015

Please cite this article as: Ruirui Liu, Wenjun Guo, Bin Sun, Jinli Pang, Meishan Pei, Guowei Zhou, Composites of rutile TiO2 nanorods loaded on graphene oxide nanosheet with enhanced electrochemical performance, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.01.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Composites of rutile TiO2 nanorods loaded on graphene oxide nanosheet with enhanced electrochemical performance Ruirui Liu,a Wenjun Guo,b Bin Sun,a,c Jinli Pang,a Meishan Pei,b and Guowei Zhou,a a

Key Laboratory of Fine Chemicals in Universities of Shandong, School of Chemistry

and Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353, P. R. China b

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School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, P.

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R. China. c

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State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, P. R.

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China.

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Graphical abstract

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Highlights

A two-phase self-assembly approach was utilized to prepare TiO2 nanorod/GO.

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TiO2 nanorods were uniformly loaded on the surface of GO nanosheets.

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The electrochemical performance of TiO2 nanorod/GO was studied systematically.

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TiO2 nanorod/GO exhibit much higher Cs and cycle stability than bare TiO2.

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TiO2 nanorod/GO composite with 4.00 wt.% GO showed the highest Cs.

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Corresponding author. Tel: +86 531 89631696.

E-mail address: [email protected] (G. W. Zhou)

Abstract

TiO2 nanorod/graphene oxide (TiO2 nanorod/GO) composites with different TiO2/GO weight ratios were successfully prepared by self-assembly of GO and ready-made TiO2 nanorods under room temperature conditions. TiO2 nanorods were synthesized via the hydrothermal method, and GO was obtained via a modified

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Hummers method. X-ray diffraction, atomic force microscopy, and high-resolution

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transmission electron microscopy indicated that rutile TiO2 nanorods were loaded on

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the GO nanosheet without obvious aggregation. The electrochemical performance of

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the TiO2 nanorod/GO composites was confirmed by cyclic voltammetry, galvanostatic

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charge–discharge, and electrochemical impedance spectroscopy in 1 mol L–1 Na2SO4

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aqueous electrolyte. The ratio of TiO2 nanorods to GO in composite materials has

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significant influence on electrochemical performance of composite electrodes. TiO2

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nanorod/GO composites with 4.00 wt.% GO have excellent electrochemical

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performance. The maximum specific capacitance of this composite electrode was 100

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F g–1 at 5 mV s–1 scan rate. The TiO2 nanorod/GO composites also exhibited good

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electrochemical stability with a capacitance degradation of less than 20% over 3000 cycles. The electrochemical performance of the as-prepared nanocomposites could be

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enhanced by increasing chemical interactions between TiO2 and GO.

Keyword: TiO2 nanorod/graphene oxide; composite materials; modified Hummers method; self-assembly; electrochemical performance

1. Introduction Since its discovery in 2004, graphene, a two-dimensional carbon nanomaterial, has been a major focus of research because of its unique physical and chemical properties [1–4]. As one of the most important derivatives of graphene, graphene oxide (GO) is an excellent carbon material with high specific surface area, good

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dispersion, and a surface rich in functional groups; thus, the material has attracted the

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interest of researchers from various fields [5–9]. GO presents an abundance of

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oxygen-containing functional groups (e.g., hydroxyl, epoxide, carbonyl, and carboxyl)

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on its basal plane and edges; these groups allow the combination of GO with other

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modified nanomaterials and provide vast opportunities for constructing GO-based

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hybrid nanocomposites. Therefore, modification or functionalization of GO by

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various methods is an important research focus [10–13].

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Given increased demands for hybrid electric vehicles and mobile electronic

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products with efficient energy storage and recent increases in traditional energy

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resource consumption, development of new materials for electrochemical storage with

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high performance has become an urgent requirement. TiO2 has been studied for application in large-scale energy storage because it is a high-capacity and high-current

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rate tolerance material with various nanostructures [14], such as nanorods [15, 16], nanotubes [17], nanofibers [18] and nanosquares [19]. However, certain problems limit the practical application of TiO2 ; the material presents low electronic conductivity within its network as well as relatively low theoretic capacity. In addition,

TiO2 nanostructures collapse after use, which causes serious decreases in performance in cycle tests. The use of nanostructured TiO2-based materials may be an efficient approach to deal with the inherent problems of supercapacitors or Li–ion batteries [20, 21]. For example, Zhu et al. utilized TiO2 nanofibers and rice grain-shaped TiO2 and their composites with carbon nanotubes as anode materials for Li–ion batteries [22]. Cherian et al. prepared (N,F)-Co-doped TiO2 by pyro-ammonolysis of TiF3. Li storage

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and cycling properties of TiO2(N,F) are much better than pure anatase-TiO2, with a

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reversible capacity of 95±3 mA h g−1, which is stable up to 25 cycles and had a

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coulombic efficiency of 98% [23].

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The synergetic effects of GO and TiO2 endow the TiO2/GO composites with

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excellent properties and improved functionalities. The combination of GO and TiO2

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offers a direct pathway for electron transfer between the electrode and electrolyte as

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well as a shortened diffusion path for insertion/extraction of alkali cations into/from

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TiO2, which results in higher capacitance and presents new avenues for research in

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energy conversion and storage. TiO2/GO composites have been fabricated through

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various processes, such as the ultrasonic [5], in-situ deposition [7], solvothermal [8],

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and self-assembly methods [9]. However, the reported preparation processes are complicated because they require high temperatures, and the assembly of TiO2

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requires the assistance of organic templates and/or guiding agents, such as surfactants [4, 24]. In this study, TiO2 nanorod/GO nanocomposites with different weight ratios of GO were prepared via facile self-assembly of GO and ready-made TiO2 nanorods on a

water/toluene interface. Although some studies on TiO2/GO composites have been reported [7, 8], only a few have focused on the use of tunable chemical properties of GO to construct TiO2/GO composites for electrochemical applications. TiO2/GO ratio may be easily controlled by increasing GO quantity during the two-phase self-assembly process. Meanwhile, the electrochemical properties of TiO2/GO composites are seriously affected by the composite ratio. The proposed method

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provides a facile and straightforward approach for loading TiO2 nanorods on GO

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nanosheets and may be readily extended to the preparation of differently shaped TiO2

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(e.g., nanowires, nanotubes, nanospheres, etc.) loaded on GO nanosheets for

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technological applications.

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2. Experimental

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2.1 Preparation of TiO2 nanorod/GO composites

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First, GO was synthesized from expanded graphite powder via a modified

Second,

TiO2

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information).

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Hummers’ method [25, 26] (details of the process are provided in the supporting nanorods

were prepared

using

CTAB

as a

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structure-directing agent and TBT as the precursor via a simple hydrothermal method

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[27, 28] (detailed process is found in the Supporting Information). Finally, TiO2 nanorod/GO composites with different TiO2/GO weight ratios were prepared by

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self-assembly of GO and ready-made TiO2 nanorods under room temperature conditions. In a typical synthesis process, 5 mg of GO was added to 20 mL of distilled water by sonicating for 1 h to obtain GO dispersions. Exactly 12 mg of TiO2 nanorods were dispersed in 10 mL of toluene with constant stirring. To ensure that GO

coordinated with Ti centers on the surface of the TiO2 nanorods, TiO2 dispersions were added to the GO dispersions and stirred for 24 h at room temperature. The resultant TiO2 nanorod/GO nanocomposites were harvested by centrifugation, washed with ethanol several times to thoroughly remove residual toluene, and free-dried overnight. Several TiO2 nanorod/GO nanocomposite samples were synthesized by varying the weight of added GO. The detailed reaction conditions are as follows:

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TiO2/2.04 wt.% GO (TGO1), TiO2/4.00 wt.% GO (TGO2), TiO2/7.69 wt.% GO

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(TGO3), TiO2/11.1 wt.% GO (TGO4), and TiO2/14.3 wt.% GO (TGO5). The final

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11.6, 12.3, 12.9, 13.3, and 14.8 mg, respectively.

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weights of prepared active materials for TGO1, TGO2, TGO3, TGO4, and TGO5 are

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2.2 Electrochemical characterization of the TiO2 nanorod/GO composites

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The samples were prepared as follows: The active material (TiO2 nanorods, GO,

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or TiO2 nanorod/GO composites), PVDF, and acetylene black were mixed at a mass

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ratio of 8:1:1 in NMP under stirring for 12 h to form a homogeneous slurry. This

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viscous slurry was then pressed on 1 cm2 nickel foam as the working electrode. The

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working electrode was dried in air, and then heated in a vacuum oven at 80 °C for 12

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h to evaporate the solvent. The weight of active material used for battery fabrication is approximately 0.8 mg. Electrochemical experiments were performed on an

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electrochemical workstation (ZAHNER Zennium; Germany) and a Blue Dutch supercapacitor test system with 1 mol L–1 Na2SO4 as the electrolyte. A typical three-electrode setup was constructed to measure the electrochemical properties of the working electrode; here, platinum was used as the counter electrode and Ag/AgCl was

used as the reference electrode. The effective electrode area of the active material was 1 cm × 1 cm. Finally, cyclic voltammetry (CV) curves of the working electrode were obtained at scan rates of 5, 25, 50, 75, 100, and 125 mV s–1 from −0.4 V to 0.6 V. Electrochemical impedance spectroscopy (EIS) measurements were performed with 5 mV signals from 0.01 Hz to 100 kHz. The obtained EIS data were fitted to an equivalent

electrical

circuit

model

using

Zview

software.

Galvanostatic

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charge–discharge (GCD) curves were obtained from −0.4 V to 0.6 V at current

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densities of 0.2, 0.5, 1, 2, and 3 A g–1.

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3. Results and discussion

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3.1 AFM

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The AFM images obtained reveal that GO presents as single (marked 1 in Fig.

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1A) or double (marked 2 in Fig. 1A) nanosheets. The height profile diagram showed

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that the thickness of a single-layer GO sheet is about 0.8 nm [10], which is nearly

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identical to the interlayer spacing (0.84 nm) of GO measured by XRD. These results

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suggest that exfoliation of graphite oxide down to single-layer GO sheets was

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achieved [11]. Fig. 1B and 1C represent the AFM images and height profile diagrams of TGO1 and TGO2, respectively. The figures show that TiO2 nanorods were loaded

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on GO sheets (marked 3 and 4 in Fig. 1B and 5 and 6 in Fig. 1C). The surface nanorods can be attributed to TiO2 nanorods. Considering the height profile diagram of the line shown in the AFM image, sharp peaks in the depth profile correspond to TiO2 nanorods; these nanorods were about 150 nm high and absent from the starting

GO sheets. The red circle in Fig. 1C represents the GO sheet in TGO2, which corresponds to the red circle in the height profile diagram. The GO (10 nm) in the hybrid was thicker than that in the primary GO sheet (Fig. 1A), likely because of stacking of single GO layers or overlaps between layers [12, 13]. The overall thickness of the TiO2 nanorods and GO nanosheets was about 150 nm (marked 4 in Fig. 1B and 6 in Fig. 1C). The

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thickness of TiO2 nanorods in the AFM images was about 140 nm, which is consistent

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with the diameter of the TiO2 nanorods observed in the HRTEM images. In summary,

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comparison of the AFM images and height profile diagrams of GO sheets, TGO1, and

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TGO2 confirms that TiO2 nanorod/GO composites were successfully obtained.

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3.2 XRD

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The crystal structure and orientation of the as-prepared samples were studied by

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XRD. Fig. 2 shows the XRD patterns of GO, TiO2, TGO1, TGO2, TGO3, TGO4, and

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TGO5. As shown in Fig. 2a, the peak at about 10.5° may be attributed to the (002)

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reflection of GO, which has an interlayer distance of 0.84 nm based on the Bragg

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equation. Furthermore, the typical (002) diffraction peaks of few layer grapheme (FLG) is approximately 27° [29], and the data were different from that of our

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prepared GO. The XRD pattern of the TiO2 nanorods is shown in Fig. 2b. Peaks at 2θ values of 27.5, 36.08, 41.23, 44.01, 54.32, 56.64, 62.74, 64.04, 69.01, and 69.79 can respectively be indexed to the (110), (101), (111), (210), (211), (220), (002), (310), (301), and (112) crystal planes of rutile TiO2. The observed data are in good

agreement with the standard rutile phase of TiO2 (JCPDS card number 21–1276). As shown in Figs. 2c–2g, TiO2 nanorod/GO composites with different weight ratios of GO exhibit the rutile crystal phase. No GO peak was observed in any of the composite samples, likely because of the relatively low content of GO in the composites; hence, GO peaks were masked by the diffraction signals of TiO2 [30]. Destruction of the regular stacking of GO during composite preparation may also explain these findings

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[7]. The (110) peak for TiO2 nanorod/GO composites is much broader than that for

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GO (Fig. 2, inset). Peak broadening suggests that the lattice structure of TiO2 is

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distorted by interaction with GO [31]. The average crystal sizes of the TiO2 and TiO2

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nanorod/GO samples were calculated using the Scherrer equation based on the XRD

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peak broadening of the (110) peak (Table 1). The sizes obtained ranged from 14 nm to

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26 nm, much smaller than the obtained size of TiO2 (27 nm). This result is caused by

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the interaction between GO and TiO2 [31].

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3.3 HRTEM analysis

The HRTEM image of the GO nanosheet is shown in Fig. 3A. The presence of

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wrinkles and folds on the sheet is characteristic of GO sheets with only a few layers. Fig. 3B shows TiO2 nanorods with typical diameters of 50–150 nm and lengths of 200–400 nm. Figs. 3C and 3D display representative HRTEM images of the as-synthesized TGO1 and TGO2, respectively; here, the GO sheets are entirely

covered by TiO2 nanorods. TiO2 nanorods exist homogeneously over the whole GO single sheet without obvious aggregation. HRTEM analysis confirmed the successful self-assembly of TiO2 nanorods on the surface of whole GO sheets. This finding is consistent with the AFM test results

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3.4 Raman measurements

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Raman spectroscopy is widely used to characterize the electronic structure and

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defects of various carbon composite materials. Fig. 4A shows the Raman spectra of

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pure GO, TiO2, and TGO2 composites taken from 100 cm−1 to 3000 cm−1. As shown in

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the inset in Fig. 4A, the characteristic peaks of GO at 1361 and 1596 cm−1 are

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ascribed to sp3 (D band)- and sp2 (G band)-hybridized carbon atoms, respectively [32].

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FLG showed D and G bands in the Raman spectrum at 1352 and 1578.6 cm−1,

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respectively [29]. The intensity and position of D and G bands of FLG are different

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from that of prepared GO. The D band suggests a common feature for sp3 defects in

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carbon, and the G band provides information on the in-plane vibration of sp2-bonded

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carbon atoms [32]. The intensity ratio of the D band to the G band usually reflects the order of defects in GO or graphene [29, 33]. Based on the Raman diagram, four

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conclusions can be drawn. First, the intensity of the G band is stronger than that of the D band, which confirms the high crystallinity and chemical stability of GO and the TiO2/GO nanocomposites [23]. The Raman features of the TiO2/GO hybrid were especially striking. The calculated ID:IG values of GO, TGO1, TGO2, TGO3, TGO4,

and TGO5 were 0.844, 0.884, 0.647, 0.710, 0.831, and 0.874, respectively (Table 1). The calculated ID:IG values of TGO2, TGO3, and TGO4 were obviously lower than that of GO, which indicate a lower density of defects and higher crystallinity in the TiO2 nanorod/GO composites [32] compared with GO. The graphitization degree increased significantly because of distortion of the GO layer by titanium atoms, and the conductivity of the TiO2 nanorod/GO composites was better than that of TiO2. In

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addition, as shown in Table 1, the calculated ID:IG of the TiO2 nanorod/GO samples

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initially decreased and then increased with increasing GO content in the composites.

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The literature reports that decreases in ID:IG are a clear indication of increases in the

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number of graphene oxide layers [34]. This finding is consistent with the AFM and

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HRTEM results. Second, the observed D (1357 cm−1) and G (1605 cm−1) bands of the

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composite were slightly shifted compared with those of GO (Figs. 4A and 4B). The

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blueshift (from 1596 cm−1 to 1605 cm−1) of the G band can be attributed to conversion

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of GO sheets into TiO2 nanorod/GO composite or resonance of isolated double bonds

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at higher frequencies [35]. Therefore, both the change in Raman band intensity and

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the blue shift of the G band provide clear evidence of the presence of GO in the

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composites. Third, Fig. 4C shows the 2D bands of GO sheets and TGO2. The 2D band, which originates from a two-phonon double-resonance Raman process, provides

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information on the stacking order of graphitic sp2 materials [36]. The change in 2D band intensity further proves that TiO2 nanorod/GO nanocomposites were successfully prepared. Fourth, the Raman spectrum in Fig. 4A contains four characteristic peaks at 152, 246, 443, and 610 cm−1, which correspond to the B1g, Eg,

and A1g modes of rutile TiO2 nanorods [22]. The composites were slightly shifted compared with pure TiO2, which further proves that composite materials were successfully prepared. Fig. 4D shows that TiO2 nanorod/GO composites with different GO contents exhibit patterns similar to the XRD patterns. However, the intensity of the TiO2 and GO characteristic peaks initially decreased and then increased with increasing weight ratio of added GO, which means a change in the crystallinity of the

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TiO2 nanorod/GO composites, as shown in Fig. 4D(a–d). Raman analysis further

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confirmed that TiO2 nanorods were effectively loaded on the GO sheet.

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3.5 Electrochemical performance of the as-prepared TiO2 nanorods, GO, and TiO2

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nanorod/GO electrodes

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The electrochemical properties of the GO, activated carbon, TiO2 nanorods, and

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TiO2 nanorod/GO composites with different GO contents were evaluated using a

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three-electrode system in 1 mol L−1 Na2SO4 electrolyte solution. CV, EIS, and GCD

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measurements were performed to compare the properties of these materials. Fig. 5A

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shows typical CV curves between −0.4 and 0.6 V at 50 mV s−1 scan rate. The CV

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curve of the TGO2 electrode approximated an ideal rectangle with a greater area and a more symmetrical shape than that of the CV curves acquired for GO and the TiO2

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nanorods. In addition, the area of the CV curves initially increased and then decreased with increasing weight ratio of GO; the area of the CV curve of the TGO2 electrode was the highest among the curves obtained. The area of the cycling curve of the activated carbon is close to that of TGO3 and TGO4. The CV curve of TGO2 electrode

is approximately an ideal rectangle with a greater area and a more symmetrical shape than that of commercial activated carbon. These results indicate that the TGO2 electrode exhibits lower resistance and more ideal supercapacitor behavior [37] than other samples. The rectangular area of the CV curves was significantly enhanced by introduction of TiO2 to the GO. This enhancement is mainly ascribed to the pseudo-capacitance of the electrochemically active TiO2.

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The CV curves for the TGO2 electrode (Fig. 5B) in the potential window of –0.4

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V to 0.6 V at various scan rates (5−125 mV s−1) show that the area defined by the CV

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curve increased with scan rate and exhibited a rectangular shape without obvious

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distortion even at scan rates of up to 125 mV s−1. These results show that the TGO2

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electrode has excellent capacitance behavior and low contact resistance. The excellent

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capacitance behavior and low contact resistance of TGO2 composites mainly provide

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a high surface area interface between the TiO2 nanorods and the electrolyte. These

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results were consistent with the BET analysis. The corresponding data are shown in

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Table 1. Table 1 shows that all TiO2/GO composite samples have larger specific

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surface areas than pure TiO2 because of the presence of GO in the composites, which

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has an extremely high surface area [6]. The BET specific surface area initially increased, and then decreased with increasing amount of GO. TGO2 has the largest

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specific surface of 133.5 m2/g, demonstrating that loading the right amount of TiO2 nanorods can effectively prevent agglomeration of GO sheets during the preparation of TiO2 nanorod/GO composites. Such high surface areas provide more surface-active sites and facilitate easy charge-carrier transport, leading to enhanced electrochemical

performance [6]. The specific capacitance (Cs) is proportional to the area under the CV curve, which is clearly much larger for the TiO2 nanorod/GO composites with different GO content than for TiO2 nanorods and GO. The Cs of an electrode can be calculated from the CV curve using the following Eq. (1) [38]:

 Idv msV

(1)

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Cs 

where Cs is the specific capacitance of the electrode (F g−1), I is the response current

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of the CV curves (mA), m is the mass of the electrode material (g), s is the scan rate

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(mV s−1), and ΔV is the potential window (V).

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Fig. 5C shows the plots of the Cs of TiO2 nanorods, GO, and TGO2 composite

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electrodes as a function of scan rate. The maximum Cs of the TGO2 electrode was 100

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F g−1 at 5 mV s−1 scan rate. The Cs values of GO and TiO2 nanorods were 31 F g−1 and

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4.8 F g−1, respectively, at 5 mV s−1 scan rate (some statistical experiment data of TiO2,

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GO, and TiO2/GO are placed in Table S1). The Cs values of GO and TiO2 electrodes

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were less than that of TGO2 electrode. Furthermore, Cs decreased with increasing scan rate. The decrease in Cs at high scan rates may be attributed to the parts of the

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electrode surface that are inaccessible to the electrolyte [39]. In general, the rate

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capability is heavily dependent on three processes [40]: ion diffusion in the electrolyte, surface adsorption of ions on the electrode materials, and charge transfer in the electrode. At a high scan rate, all of the three processes are relatively slow, which limits the rate and lowers the Cs. When the scan rate reached 125 mV s−1, 73% capacitance remained in TGO2, which indicates the higher Cs of the TiO2 nanorod/GO

composite electrode compared with that of a TiO2 electrode reported previously. For example, Salari et al. prepared self-organized TiO2 nanotube array supercapacitor electrodes that had a Cs of 19.2 F g−1 at 1 mV s−1 by anodic oxidation [41]. Ramadoss et al. prepared TiO2 nanorod electrodes via a hydrothermal method; these electrodes exhibited a maximum Cs of 8.5 F g−1 at a scan rate of 5 mV s−1 [42].

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EIS spectra were also obtained to characterize the electrode properties. Fig. 6

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reveals the Nyquist plots of GO and TiO2/GO composites with different GO content

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electrodes. Each Nyquist plot contains a semicircle in the high-frequency region and a

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straight line in the low-frequency region. The semicircle arc in the high-frequency

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region may be related to electronic resistance, and the vertical line in the

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low-frequency region indicates pure capacitive behavior [40]. The more vertical shape

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at low frequencies for TGO2 indicates the increasingly capacitive behavior of

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electrodes. The inset in Fig. 6 shows the Nyquist plot of the TiO2 nanorod electrode.

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The TiO2 Nyquist plot does not contain a semicircular region, which is probably due

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to the low Faradaic resistance of the electrode and the high electrical conductivity

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between the nanorods and the current collector [41]. The bottom inset in Fig. 6 is the equivalent circuit [43, 44], in which Rs is the

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total resistance of the electrolyte, separator, and electrical contacts, Rct is the charge-transfer resistance, Zw is Warburg impedance, and Cdl represents the double-layer resistance. Rs is derived from the high-frequency intersection of the Nyquist plot in the real axis, whereas the diameter of the semicircle corresponds to Rct

of the electrodes and electrolyte interface [44]. The linear region (Zw) of the plot exhibits an angle between 45° and 90° relative to the real axis, indicating that the electrode process is not perfectly capacitive in nature, but under diffusion control. At high frequencies, Rct values of TiO2/GO composites were obviously lower than that of GO, confirming that the electronic conductivity of GO improved after loading of TiO2. The Rct values initially decreased, and then increased with decreasing TiO2 loading.

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Rct of TGO2 composite was minimal. Rct of TGO2 composite was estimated to be 2.8

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Ω from the semicircular arc in the high-frequency region. This small Rct is attributed

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to the following: (1) aggregation of GO nanosheets was prevented by the TiO2

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nanorods and (2) intimate contact between highly dispersed TiO2 nanorods and GO

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sheets, which minimizes the interfacial resistance of the charge transfer process.

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These results demonstrate that the enhanced electrochemical performance of the

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hybrid material could be ascribed to synergistic effects of the TiO2 nanorods and GO

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sheets.

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Based on the test results presented above, the improved electrochemical

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performance of TGO2 was further confirmed by GCD tests performed under different current densities. Fig. 7A shows the GCD curve obtained from the TGO2 electrode in

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1 mol L−1 Na2SO4 at a current density of 2 A g−1 from −0.4 V to 0.6 V. The curve is symmetric and resembles an equicrural triangle, which is indicative of a highly efficient charge–discharge process. Fig. 7B shows the GCD curves for the TGO2 system at five different current densities of 0.2, 0.5, 1, 2, and 3 A g−1. The linear

voltage–time profile and the highly symmetric charge–discharge characteristics are indicative of the good capacitive behavior achieved by the TGO2 system [2]. Based on the charge–discharge curve, the Cs of the electrode can be calculated using the following Eq. (2) [38]: Cs 

It mV

(2)

where I is the discharge current, Δt is the discharge time, m is the mass of the

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electro-active material, and ΔV is the potential window. The calculated Cs values of

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the TGO2 electrode were 78, 72, 65, 64, and 60 F g−1 at 0.2, 0.5, 1, 2, and 3 A g−1,

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respectively. These values are mainly consistent with the order indicated by the CV

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curves. The Cs values obtained in this work were generally higher than those obtained

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in other recent reports on TiO2 [45, 46]. The Coulombic efficiency (η), energy density

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(E) and power density (P) of the electrode were calculated from charge/discharge

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curves (see Supporting Information) [22].

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Given that a long cycle life is among the most important criterion for a

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supercapacitor, an endurance test was conducted using GCD cycles at 0.2 A g−1 (Fig.

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7C). Fig. 7C shows that the Cs retention ratio of the TGO2 electrode was 80% even

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after 3000 cycles, which indicates excellent cycle stability. However, in previous reports, the Cs retention ratio of TiO2 electrodes was only 80% after 1000 cycles [44].

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Hence, three effects of the GO component in promoting the high capacity and long-cycle life of TGO2 composites are proposed. First, GO inhibits aggregation of TiO2 nanorods, which preserves the high interface surface area between the nanorods and the electrolyte. Second, TiO2 nanorod arrays act as the infrastructure that bridges

GO nanosheets and prevents them from severe swelling and shrinkage during the cycling process. Third, excellently aligned nanorods can provide well-ordered tunnels, which are convenient for insertion/extraction of alkali cations into/from TiO2.

4. Conclusions In summary, TiO2 nanorod/GO nanocomposites were prepared with different weight ratios of GO via facile self-assembly of GO and ready-made TiO2 nanorods on

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a water/toluene interface. The TiO2 nanorods obtained were uniformly loaded on the

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surface of GO nanosheets. The electrochemical performance of the TiO2 nanorod/GO

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nanocomposites was determined, and results showed that the nanocomposites exhibit

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much higher specific capacitance and cycle stability than bare TiO2. The influence of

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GO on the electrochemical performance of the TiO2 nanorod/GO nanocomposites was

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examined systematically by considering the different weight ratios of GO added to the

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products. The Cs of the TiO2 nanorod/GO samples initially increased and then

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decreased with increasing GO content in the composites. Among the samples studied,

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the TiO2 nanorod/GO composite with 4.00 wt.% GO showed the highest Cs (100 F g–1)

EP

at a scan rate of 5 mV s–1. Enhancements in electrochemical performance are

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attributed to the synergistic effect of TiO2 and GO. This work demonstrates that careful design of GO-based composites by coupling of the material with multiple

A

semiconductor compounds contributes to the development of next-generation supercapacitor systems with significantly improved electrochemical performance.

Acknowledgements This work was supported by the National Natural Science Foundation of China

(Grant No. 20976100, 51372124), the Natural Science Foundation of Shandong Province (Grant No. ZR2011BQ009), and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province.

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Table 1. Average crystal sizes, BET surface area and the caculated ID: IG from raman

Average crystal size

BET surface area

(nm)

(m2/g)

GO

-

80.7

0.844

TiO2

26.9

19.8

-

TGO1

15.4

87.9

0.884

TGO2

14.4

133.5

TGO3

25.8

110.3

PT

spectra.

TGO4

24.8

113.5

TGO5

22.3

N A M D TE EP CC A

I D: I G

0.647

RI

0.710

SC 120.2

U

Sample

0.831 0.874

Figure Captions Fig. 1. AFM images and height profile diagrams of (A) GO, (B) TGO1, and (C) TGO2. Fig. 2. XRD patterns of (a) GO, (b) TiO2 nanorods, (c) TGO1, (d) TGO2, (e) TGO3, (f) TGO4, and (g) TGO5. Inset XRD pattern showing the 25−30 degrees region. Fig. 3. HRTEM images of (A) GO, (B) TiO2 nanorods, (C) TGO1, and (D) TGO2.

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Fig. 4. (A) Raman spectra of (a) TiO2 nanorods, (b) TGO2, and (c) GO. (B) and (C) a

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larger version of G band and D band of the corresponds GO and TGO2. (D)

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Raman spectra of (a) TGO1, (b) TGO3, (c) TGO4, and (d) TGO5.

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Fig. 5. (A) CV curves of TiO2 nanorods, active carbon, GO, and TiO2/GO composites

N

with different GO content electrodes at a scan rate of 50 mV s−1 in a 1 mol L−1

A

Na2SO4 electrolyte solution. (B) CV curves for the TGO2 electrode in the

M

potential window of –0.4 V to 0.6 V at various scan rates (5−125 mV s−1). (C)

D

the plots of the Cs of TiO2 nanorods, GO, and TGO2 composite electrodes as a

TE

function of scan rate.

EP

Fig. 6. Nyquist plots of GO and TiO2/GO composites with different GO content

CC

electrodes. The inset shows the Nyquist plot of the TiO2 nanorods electrode and the equivalent circuit used to model the impedance spectra.

A

Fig. 7. (A) GCD curve of the TGO2 electrode in 1 mol L−1 Na2SO4 at a current density of 2 A g−1. (B) GCD curves of the TGO2 electrode at five different current densities of 0.2, 0.5, 1, 2, and 3 A g−1. (C) cycle stability of the TGO2 electrode at a current density of 0.2 A g−1.

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EP

CC

A D

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RI

SC

U

N

A

M

Fig1 .

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EP

CC

A D

Fig. 3

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RI

SC

U

N

A

M

Fig2 .

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EP

CC

A D

PT

RI

SC

U

N

A

M

Fig4 .

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EP

CC

A D

.

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RI

SC

U

N

A

M

Fig5

TE

EP

CC

A D

PT

RI

SC

.

U

N

A

M

Fig6

TE

EP

CC

A D

PT

RI

SC

U

N

A

M

Fig7 .

A

CC

EP

TE

D

M

A

N

U

SC

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Graphical Abstract .