TiO₂ nanotubes grown on graphene sheets as advanced anode materials for high rate lithium ion batteries

Abstract

Here, we propose a unique architecture based on directly growing TiO₂ nanotubes on graphene sheets. This work establishes the benefits of combining nanotubular TiO₂ with conductive graphene to achieve a significant enhancement in capacitive energy storage. The TiO₂ nanotubes possess a porous structure and a higher surface area compared with its bulk material. The graphene sheets play an important role in the conductive substrate for charge collection. In the preparation process, amorphous TiO₂ colloids coated on graphene oxide (GO) sheets serve as the seeds to realize the growth of nanotubes, and ensure the robust contact between graphene and TiO₂ nanotubes to benefit charge transfer. When used as anodes for lithium ion batteries, the graphene–TiO₂ nanotubes (Gr–TNTs) manifest excellent rate capability and cycling performance, indicating that the nanotubes, graphene and their synergistic effect can prompt fast charge transfer in such a composite structure.

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Introduction

Graphene possesses superior electrical conductivity, a large specific surface area, strong mechanical stability, and chemical stability.^1,2 Designing and fabricating graphene-based nanocomposites with highly specific morphology and composition has become a hot topic for research in modern materials chemistry because of their new and/or enhanced functionalities, which cannot be achieved by either component alone, and their wide application in electronic, photocatalytic and energy storage fields.^3,4 In this regard, remarkable progress has been made in hybrid nanocomposites consisting of graphene with functional materials, such as metals oxide,^5–7 copolymers,^8,9 and biomaterials.¹⁰

Titanium oxide is one of the most intensively investigated transition-metal oxides as a lithium-ion host because of its high theoretical capacity, high safety, environmental benignity, etc.^11–13 A challenging issue in using TiO₂ for lithium-ion batteries is to tackle its poor electronic conductivity. Early research work manifested that the introduction of a carbon-based conductor or nanoarchitectured electrodes is an effective strategy to improve electronic conduction. Owing to its superior conductivity and larger surface area than a conventional carbon conductor, a great effort has recently been focused on constructing graphene-based hybrid composites with nanostructured TiO₂ (including nanoparticles, nanorods and nanosheets) for improving lithium storage performance.^13–15 Although many kinds of graphene–TiO₂ nanocomposites have been made, it is hard to build up a good interface between graphene and TiO₂ nanotubes, which is required for rapid ion diffusion and electron transition during the Li⁺ insertion–extraction process.

Herein, a unique architecture of graphene-based hybrid TiO₂ nanotubes was fabricated using GO sheets decorated with TiO₂ colloids as a precursor. TiO₂ colloids on GO, served as seeds to initialize the growth of TiO₂ nanotubes and ensured the tight bonding between the graphene sheets and nanotubes. Because of the short ion diffusion path of nanotubes and the high conductivity of graphene, the Gr–TNTs nanocomposites should be an ideal anode material for high rate lithium ion batteries. Galvanostatic testing results demonstrate that the nano-architectured Gr-TNTs show enhanced Li⁺ insertion–extraction kinetics in TiO₂, especially at high charge–discharge rates.

Experimental section

Preparation of Gr–TNTs nanocomposites

For the fabrication of Gr–TNTs, TiO₂ colloids coated on graphene oxide sheets were first prepared by the controlled hydrolysis of titanium tetraisopropoxide (TTIP, 97% Aldrich) in the presence of GO sheets. Then the TiO₂ colloids-contained GO sheets were in situ transformed into Gr–TNTs by a hydrothermal process followed by calcination. Typically, 40 mg of GO, fabricated using the modified Hummers method,¹⁶ was dispersed into 20 mL of an ethanol–water (50 : 1) solution, followed by the addition of 2 mmol of TTIP with stirring. After being stirred for 20 min, 2 mL of distilled water was added to ensure complete hydrolysis of TTIP. The TiO₂ colloids-contained GO deposited at the bottom of the vessel, which was collected and then mixed with 20 mL of NaOH (0.1 M) and H₂O₂ (0.04 wt%) solution. The suspension was transferred into a 30 mL autoclave and then placed in an oven at 180 °C for 8 h. After the hydrothermal reaction, the products were neutralized using 0.1 M HCl and subsequently washed with deionized water to pH = 7, dried at 60 °C under vacuum for 3 h, and finally treated at 400 °C in H₂/Ar for 4 h with a heating rate of 1 °C min⁻¹ to obtain the Gr–TNTs. For comparison, TiO₂ nanotubes were prepared by a similar process without the addition of graphene.

Characterization

The structure and morphology of the products were characterized by X-ray diffraction measurements (XRD, Rigaku, D/max-RB using Cu Ka radiation), transmission electron microscopy (TEM, JEOL JEM-2010 equipped with an energy dispersive X-ray detector) and field emitting scanning electron microscopy (FESEM, JEOL JSM-7401F), respectively. High resolution TEM was performed on a JEOL JEM-2100F with an acceleration voltage of 200 kV. The N₂ sorption studies were carried out at 77 K using a Micromeritics ASAP 2000 instrument. Raman spectra of the samples were obtained with Raman spectroscopy with a laser excitation energy of 532 nm. Thermogravimetric analysis (TGA) was conducted on a TA instruments Q50 by heating from room temperature to 700 °C at 5 °C min⁻¹ in a nitrogen atmosphere.

Electrochemical characterization

The charge and discharge capacities were measured with coin cells in which a lithium metal foil was used as the counter electrode. The electrolyte employed was a 1 M solution of LiPF₆ in ethylene carbonate and dimethyl carbonate (EC + DMC) (1 : 1 in volume). The Gr–TNTs powder (80 wt%), acetylene black (10 wt%) and polyvinylidene fluoride binder (10 wt%) were homogeneously mixed in NMP solvent with magnetic stirring (For pure TiO₂ nanotubes, the ratio is 65 : 25 : 10). After stirring for 3.5 h, the slurry was coated uniformly on aluminum foil. Finally, the electrode was dried under vacuum at 110 °C for 10 h. Cell assembly was carried out in an argon-filled glove box (German, M. Braun Co., [O₂] < 1 ppm, [H₂O] < 1 ppm). The coin cells were cycled under different current densities between cut off voltages of 3.0 and 1.0 V on a CT2001A cell test instrument (LAND Electronic Co.) at room temperature.

Result and discussion

In a previous report, we proposed a mild hydrothermal approach to prepare TiO₂ nanotubes. In that process, TiO₂ colloids are used as seeds to initialize the growth of sodium titanate nanotubes in a low concentration solution of NaOH and H₂O₂, and the sodium titanate nanotubes converted to TiO₂ nanotubes after acid washing and heat treatment. Our investigation reveals that H₂O₂ plays an important role in the formation of titanate nanotubes, since only titanate nanosheets are formed without the addition of this agent.¹⁷ With the assistance of H₂O₂, the titanate nanosheets can scroll into nanotubes even in a low concentration solution of NaOH (0.1 M). In the present work, we further developed this method to grow TiO₂ nanotubes on graphene sheets.

As illustrated by Fig. 1a, GO sheets with tens of micrometers are used as the support for growth of TiO₂ nanotubes. Since the functional groups of GO provide nucleation centers to initialize the seeds of TiO₂, GO sheets can be uniformly coated by a layer of TiO₂ colloids from the hydrolysis of TTIP in ethanol (Fig. 1b). Theses TiO₂ colloids on GO could guide the growth of titanate nanotubes. During the hydrothermal process, homogenous tiantate nanotubes grow on the GO sheets to form a 2D plate-like structure (Fig. 1c and d and S1 in the ESI†). After acid washing and calcination in H₂/Ar, the 2D plate-like GO–titanate nanotubes composites transform to Gr–TNTs without their structure being destroyed. As shown in Fig. 2a, a piece of graphene is completely covered by TiO₂ nanotubes. Such 2D plate-like structure of Gr–TNTs demonstrates the successful growth of TiO₂ nanotubes on the graphene sheets, since only aggregation of TiO₂ nanotubes will be formed without the graphene support (Fig. S2 in the ESI†). The detailed morphology of the 2D structure of Gr–TNTs (Fig. 2b) shows that a dense mass of TiO₂ nanotubes is uniformly distributed on the graphene sheets. These TiO₂ nanotubes are apart from each other and the aggregation of nanotubes is retarded. Fig. 2c and d show the TEM images of Gr–TNTs, which further confirm that the roots of TiO₂ nanotubes are firmly fixed on the graphene support. We suppose that the tight contact between nanotubes and graphene is favorable to improve the lithium storage performance. It can also be seen in Fig. 2d that the length of TiO₂ nanotubes is about 200 nm, and the inner diameter is ca. 20 nm.

Fig. 1 SEM images of (a) GO sheets, (b) GO sheets coated by TiO₂ colloids, (c and d) SEM and TEM images of GO sheets supported titanate nanotubes.

Fig. 2 (a and b) Scanning electron microscope images of Gr–TNTs; inset in (b) high-magnification image showing TiO₂ nanotubes on the graphene sheets; (c and d) transmission electron microscope images of the Gr–TNTs.

X-ray diffraction (XRD) is used to monitor the structure and composition of the samples. As displayed in Fig. 3a, the diffraction peak at 10.6° corresponds to the (002) diffraction peak of GO.¹⁸ After hydrothermal reaction and following heat treatment, the typical diffraction peak of GO disappears, whereas a small and low broad (002) diffraction peak appears at 26°, which indicates that GO has been successfully reduced to graphene.¹³ For Gr–TNTs, all the diffraction peaks are ascribed to the anatase phase of TiO₂. There are no obvious diffraction peaks of graphene observed in the pattern of Gr–TNTs, suggesting that graphene sheets are homogeneously dispersed and coated with TiO₂ nanotubes.¹⁸

Fig. 3 X-ray diffraction patterns of graphene, graphene oxide and the Gr–TNTs.

The graphene in the Gr–TNTs can be determined by Raman analysis. Fig. 4 shows the typical Raman spectra of anatase TiO₂ and the Gr–TNTs. The TiO₂ spectra display three Raman-active fundamentals in the vibrational spectrum: E_g modes centred around 639 cm⁻¹, and two B_1g modes at 399 and 519 cm⁻¹. Besides, the Raman-allowed phonon peaks of TiO₂, two peaks at higher frequencies (1331 cm⁻¹ and 1592 cm⁻¹) observed in the spectra of the Gr–TNTs can be assigned to the typical D-band and G-band peaks of graphene.⁷ The D/G intensity ratio of the Gr–TNTs decreased when compared to that in the spectrum of GO–TiO₂, which suggests an increase in the average size of the in-plane sp² domain upon the reduction of GO to graphene.

Fig. 4 Raman spectra of the Gr–TNTs, anatase TiO₂ and GO–TiO₂ before the hydrothermal process.

The oxygen content of GO before and after reduction was analyzed by XPS (Fig. S3 in the ESI†). The oxygen content of GO was estimated to be 38%, while after heat-treatment at 400 °C in H₂/Ar for 4 h the percentage of oxygen decreased to 5.6%. FT-IR (Fig. S4 in the ESI†) shows the characteristic peaks of GO appear for O–H (3400 and 1625 cm⁻¹), C=O (1721 cm⁻¹), epoxy C–O (1219 cm⁻¹) and alkoxy C–O (1053 cm⁻¹) groups.¹⁹ Although the peaks for the C=O groups are reduced significantly after reduction, the residual peaks for the alkoxy C–O groups are still found with a small shift from 1053 cm⁻¹ to 1110 cm⁻¹. These observations suggest that C–O functional groups may link to the TiO₂ nanotubes, indicating the possible presence of C–O–Ti bonds between TiO₂ and graphene. Similar deduction has also been done in previous reports on magnetite/graphene nanosheet composites.²⁰ In addition, the peaks of Gr–TNTs at 500–900 cm⁻¹ can be indexed to the Ti–O–Ti stretching vibration modes in the TiO₂.²¹ The TG analysis indicates that the content of TiO₂ in the Gr–TNTs is about 84.5 wt% (Fig. S5 in the ESI†).

The specific surface area and pore structure of the Gr–TNTs was analyzed by nitrogen adsorption and desorption (shown in Fig. 5). The isotherm is a typical type IV-like with type H3 hysteresis loop, indicating the presence of mesoporous material. The Barrett–Joyner–Halenda (BJH) pore-size distribution curve indicates that the average pore diameter is 18.5 nm, which is consistent with the TEM observations. In addition, the Brunauer–Emmett–Teller (BET) surface area is 105 m² g⁻¹. It is well known that the large specific surface area and mesoporous structure of nanotubes are favorable for rapid lithium-ion insertion–extraction in electrode materials. Furthermore, the graphene substrate can enhance the electronic conductivity of TiO₂. When used in lithium ion batteries, the high ionic and electronic conductivity will delay the capacity loss associated with the concentration polarization to high current density. Thus, the Gr–TNTs might be an ideal system of choice for application in high rate lithium ion batteries.

Fig. 5 Nitrogen adsorption–desorption BET isotherm and the pore size distribution of the Gr–TNTs.

Recently, extensive work has been made in exploration of various graphene–nano-TiO₂ composites in order to enhance capacity and cycle performance. Beyond that, the rate performance of TiO₂ also needs to be improved in the light of future battery applications in high-power devices. The voltage profiles of Gr–TNTs composite for charging to 3.0 V and discharging to 1.0 V at increasing rates from 0.5 C to 10 C (1 C = 337 mA h g⁻¹) are shown in Fig. 6a. They demonstrate that the Gr–TNTs nanocomposite can sustain excellent charge and discharge rates. At the low rate of 0.5 C, the discharge process of the sample consists of three stages; the first stage is the quick voltage drop, the second stage is the distinct voltage plateau, and the third stage is a gradual decay in potential. The voltage plateau is related to the phase transition between the tetragonal (I4₁/amd) and orthorhombic (Li_0.5TiO₂, space group Pnm2₁) phases with Li insertion into anatase TiO₂. The voltage plateau decreases with an increasing current density. It is 1.75 V at 0.5 C and drops to about 1.25 V at 10 C. Moreover, the specific capacity exhibits a tendency to decrease with increasing current density.

Fig. 6 (a) The first charge–discharge curves and (b) rate performance of the Gr–TNTs composite between 1.0 V and 3.0 V at different current densities.

The rate performance of the Gr–TNTs at different current densities is illustrated in Fig. 6b. It can be seen that a specific capacity of 223 mA h g⁻¹ is obtained at 0.5 C after 5 cycles, and the value decreases gradually to 180 mA h g⁻¹ at 1 C after 10 cycles, and 145 mA h g⁻¹ at 5 C after 10 cycles. Even at the high rate of 10 C, a capacity of 118 mA h g⁻¹ can still be delivered, which is not only superior to the capacity of bare TiO₂ nanotubes (80 mA h g⁻¹ at 10 C) but also higher than that of previously reported graphene–TiO₂ electrodes.^{13–15,22–25} The large capacities of Gr–TNTs certify the much enhanced lithium storage performance of TiO₂ nanotubes by incorporation of graphene sheets. When the charge–discharge current density returns to 1 C, the capacity of Gr–TNTs can be restored to 178 mA h g⁻¹, showing a high reversibility. The excellent rate performance of Gr–TNTs should be ascribed to its unique hybrid structure and the synergistic effect of the components. On the one hand, the nanotubes with interspacing have a higher electrode–electrolyte contact area than aggregated TiO₂ nanotubes, which provides more channels for Li-ion insertion–extraction and electron transition. On the other hand, the graphene sheets play an important role as a highly conductive substrate for current collection, which enhances the electron conductivity of the whole electrode. Besides, the tight bonding between the nanotubes and graphene sheets, which facilitates fast electron-transport to the current collector and enhances the rate capability of TiO₂.

We also investigated the cycle behavior of the Gr–TNTs at the high rate of 5 C, as shown in Fig. 7. In the first cycle, the capacity of the sample decreases significantly from 176 to 159 mA h g⁻¹. The serious capacity loss can be ascribed to irreversible Li insertion sites in TiO₂ and the adsorbed trace water in the electrode, respectively.²⁶ After the initial capacity loss, the Gr–TNTs composite shows very high capacity retention upon cycling. The specific capacity stays above 140 mA h g⁻¹ after 150 cycles and the average capacity loss from the 2^nd to 150^th cycles is ca. 0.12 mA h g⁻¹ per cycle. The exceptional cycle stability should be ascribed to the graphene sheets in the hybrid structure, which can accommodate the strains of TiO₂ during lithium-ion insertion–extraction.

Fig. 7 The cycle performance of the Gr–TNTs at 5 C in the potential range of 3.0–1.0 V.

Conclusions

In summary, we have designed a graphene-based hybrid structure by directly growing TiO₂ nanotubes on graphene sheets. The synthesis of the Gr–TNTs is based on the hydrothermal conversion of TiO₂ colloids-contained GO to TiO₂ nanotubes on graphene. By taking the advantages of nanotubes and highly conductive graphene, the unique hybrid structure is favorable to rapid ion diffusion and electron transition. The electrochemical testing demonstrates the Gr–TNTs composite as a promising anode material for lithium ion batteries with high specific capacity, excellent rate performance, and good cycle stability.

Acknowledgements

This work is supported by National 973 & 863 Program of China Grant no. 2009CB939900 & 2011AA050505, NSF of China Grant no. 21203234 & 11274328 & 51125006 & 91122034 & 51121064 & 51102263 & 21101164, NSF of Shanghai Grant 12XD1406800 & 12JC1409000, and CAS/SAFEA International Partnership Program for Creative Research Teams are acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: XRD, TEM, SEM and EDS analysis of samples. See DOI: 10.1039/c4ra05027d

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