Atomistic Simulation Studies of TiO2 Nanosheets

Studies on TiO₂ nanosheets for energy storage and conversion, including anode in lithium ion batteries (LIB) and electrodes in dye solar cells respectively, are not as abundant as those of nanoparticle and nanoporous structures [1]. Such nanosheets are ideal owing to exposure of highly reactive surfaces, such as the anatase {001}facets, which can now be stabilised [2]. Electrochemical investigations reveal that the exposed (001) high-energy facets of the nanosheet result in enhanced rate capability which originates from both the shortened diffusion path and lowered insertion energy barriers on the active surface for Li⁺ ions [3]. Furthermore, the electrolyte/electrode contact is enhanced via hollow structures; and such features collectively allow for more efficient lithium diffusion in anodes of LIB. In addition to the anatase polymorph [4], TiO₂–B [5] nanosheets have also been observed.

Generally, studies of nanostructures with the brookite structure are scarce since this polymorph is difficult to grow. Aggregations of nanoparticles with the brookite structure have been reported  [6]. However, TiO₂ nanosheets with the brookite and rutile components have been recently observed [7]. In order to understand such nano-architecture better, we present results of  TiO₂ simulated nanosheets, which were generated by amorphisation recrystallisation method [8]. Simulated X-ray diffractions of such structures allude to the presence of the brookite and rutile polymorphs. In addition analysis of their microstructures clearly show zigzag patterns associated with the brookite and straight tunnels related to twinned rutile polymorphs. The lithiated nanosheets of TiO₂ were also investigated, and lithium ions were located well within the observed tunnels. The suitability of this TiO₂polymorph and nano-architecture for lithium ion batteries electrodes, as compared to the bulk form, will be discussed.

 References

[1] Su X., Wu Q-L., Zhan X., Wu J., Wei S. and Guo Z. (2012) J. Mater Sci. 47,2519.

[2] Yang H. G., Sun C. H., Qiao S. Z., Liu J. Z. Smith S. C. Cheng H. M. and Lu G. Q. (2008) Nature 453, 638.

[3] Knauth P. and Tuller H. L. (1999) J. Appl. Phys. 85, 897.

[4] Wei X., Liu J., Chua Y. Song J. and Liu X. (2011) Energy Environ. Sci. 4,2054.

[5] Liu S.,Jia H., Han L., Wang J., Gao P., Yang J. and Che S. (2012) Adv. Mater. 24,3201.

[6] Reddy M. A., Pralong V., Varadaraju U. V. and Raveau B. (2008), Electrochem. Solid-State Lett. 11, A132 – A134.

[7] Sussman M., Clouteau, Yasin A., Guo F., and Demopoulos G.P. (2013) Abstract #881, 224th ECS Meeting, © 2013 The Electrochemical Society.

[8] P.E.Ngoepe, R.R. Maphanga and D.C. Sayle, (2013), “Towards the Nanoscale”, Chapter 9 in Computational Approaches to Energy Materials, pp 261-290, edited by C.R.A. Catlow, A. Sokol and A. Welsch, John Wiley and Sons Ltd.