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