Electrochemical and Raman spectroscopy identification of morphological and phase transformations in nanostructured TiO2(B)

Abstract

TiO₂(B) exhibits unique electrochemical lithiation behavior where two closely spaced reduction and oxidation peaks, referred to as “double peaks,” are observed for 3-D forms of bulk and nanostructured TiO₂(B) while a single broad redox peak is observed for 2-D nanosheet architectures. In this study, we have used a combination of transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Raman spectroscopy and cyclic voltammetry on TiO₂(B) nanosheets as well as a series of thermally annealed nanosheets to map the morphological and phase transformation pathways that help clarify the structure-dependent lithiation behavior. We found that the double peak redox behavior only arises once a three dimensional nanocrystalline structure of TiO₂(B) exists by observing nanoparticle growth on the TiO₂(B) nanosheet surface via TEM at temperatures above 150 °C. This morphological transformation was also verified by Raman spectroscopy. The appearance of low-energy torsional modes at temperatures above 150 °C which are not observed in 2-D morphologies of TiO₂(B) agrees well with TEM evidence of 3-D nanoparticle formation. Using scan rate dependent cyclic voltammetry we also verified that all lithiation behavior associated with TiO₂(B) (either nanosheet or nanoparticle) is due primarily to a surface redox (pseudocapacitive) mechanism. The thermal annealing study also shows the phase transformation of surface nucleated TiO₂(B) nanoparticles to anatase nanoparticles at temperatures above 200 °C. These studies clearly show how nano-morphological control can influence electrochemical lithiation behavior and help identify a possible mechanism to explain the double peak phenomenon observed for TiO₂(B).