One-pot solvothermal synthesis of graphene-supported TiO2 (B) nanosheets with enhanced lithium storage properties
Introduction
The ever-increasing energy demands have triggered significant research efforts on the development of new electrode materials for advanced energy storage devices. Among them, rechargeable lithium ion batteries as the most promising energy storage technology are currently being used to power portable electronic devices such as laptop computers, cell phones, and digital cameras [1], [2], [3], [4], [5], [6], [7]. The performance of lithium ion batteries is determined mainly by anode and cathode materials [8], [9]. Therefore, exploring new electrode materials and designing innovative nanostructures have been necessary to meet the ever-growing performance demands. Titanium dioxide, an important member of the transition metal oxide (TMO) family, is believed to be a promising candidate because of their excellent rate capability and better safety than those of conventional graphite, as well as non-toxicity and low material cost. Therefore, various polymorphs of titanium dioxide such as rutile, [10] anatase, [11], [12], and TiO2 (B) [13], [14], [15], [16], [17] have recently been studied as electrode materials for lithium ion batteries. In particular, the polymorph of TiO2 (B) shows a more open channel structure and characteristic pseudocapacitive behavior, resulting in easier Li-ion access to the crystal structure and faster charge–discharge capability than anatase or rutile. However, the cycling performance of TiO2 electrodes at high charge–discharge rates is still poor owing to the poor electron transport in TiO2 and the aggregation tendency of TiO2 nanostructures [18]. In this regard, carbonaceous materials are commonly introduced as matrices to improve the structural stability of the electrodes [19], [20], [21], [22].
Graphene, which is a monolayer of carbon atoms arranged in a honeycomb network, is becoming one of the most appealing matrices because of its unique properties [23], [24], [25]. More recently, promising results have been reported on TMO-graphene composites, including Fe3O4 [26], [27], [28], Co3O4 [29], [30], [31], Mn3O4 [32], TiO2 [33], [34], and SnO2 [35], [36], [37]. Despite these promising results, the nanoparticles incline to agglomerate and separate from the surface of graphene because of poor interfacial contact with the graphene surface. In contrast to nanoparticles-on-graphene material, the nanosheets-on-graphene material could provide strong and stable interfacial contact with graphene surface, leading to the improvement of interfacial charge transport in the electrode. TiO2 (B) nanosheets are an important class of lithium storage materials because lithium ions can be inserted and stored between the weakly interacting layers. To sufficiently utilize the two-dimensional nature of graphene and TiO2 (B) nanosheets, we report for the first time a facile approach toward graphene-supported TiO2 (B) nanosheets (GTBN) composite for use in Li-ion batteries via a simple one-pot solvothermal process. The overall synthetic procedure of GTBN is shown in Scheme 1. Graphene oxide (GO) has abundant oxygenate groups on their surface, which are essential for the efficient adsorption of Ti3+ ions. We choose TiCl3 as the precursor for two reasons: (1) compared with TiCl4, the TiCl3 is more stable in air; (2) as a strong reducing agent, TiCl3 can reduce GO to graphene [38]. When the solvothermal reaction has occurred, TiO2 (B) nanosheets can be in situ produced via one-pot hydrolysis reaction of Ti3+ adsorbed on the surface of GO nanosheets along with the reduction of GO to graphene simultaneously.
Section snippets
Chemicals
Graphite was purchased from Alfa Aesar (325 mesh). Titanium (III) chloride (20% in 3% hydrochloric acid) and ethylene glycol were purchased from Beijing Chemical Reagent Factory. Other reagents were of analytical grade and were used as received without further purification. All aqueous solutions were prepared with Milli-Q water (18.2 MΩ cm).
Apparatus
Raman spectra were acquired on a Renishaw (Renishaw, United Kingdom) 2000 model confocal microscopy Raman spectrometer with 514.5 nm wavelength incident laser
Results and discussion
The morphology of GTBN was first examined by transmission electron microscopy (TEM). Fig. 1 panels A–C show TEM images of GTBN hybrid nanostructures at different magnifications. As shown in TEM image with a low magnification (Fig. 1A), the surface of graphene is almost covered with TiO2 (B) nanosheets. The formation of such structures implies the successful growth of TiO2 (B) nanosheets-on-graphene, since only aggregation of TiO2 (B) nanosheets with flower-like morphology is formed without the
Conclusion
In summary, we have successfully synthesized a novel nanosheets-on-nanosheets hybrid nanostructure by a solvothermal method. The hybrid material is constructed from two-dimensional TiO2 (B) nanosheets combined with two-dimensional graphene. Aside from the enhancement of electron transport, graphene has an added advantage of suppressing TiO2 (B) nanosheets aggregation during cycling, which significantly improve the diffusion efficiency of lithium ions and thus advance the rate performance and
Acknowledgments
This work was supported by the National Basic Research Program of China (973 Program, No. 2011CB935800)
References (41)
- et al.
J. Power Sources
(2012) - et al.
Electrochim. Acta
(2012) Chem. Rev.
(2004)- et al.
Angew. Chem. Int. Ed.
(2008) - et al.
Adv. Mater.
(2012) - et al.
Acs Appl. Mater. Interfaces
(2011) - et al.
J. Am. Chem. Soc.
(2011) - et al.
Science
(2011) - et al.
Acs Appl. Mater. Interfaces
(2012) - et al.
Angew. Chem. Int. Ed.
(2013)