AVS 60th International Symposium and Exhibition
    Surface Science Tuesday Sessions
       Session SS+AS-TuM

Paper SS+AS-TuM1
Low-Energy Electron Microscopy (LEEM) Studies of Langmuir-Blodgett (LB) Deposited Titania Films for Electrochemical Energy Storage

Tuesday, October 29, 2013, 8:00 am, Room 201 A

Session: Synthesis, Structure and Characterization of Oxides
Presenter: C.K. Chan, Sandia National Laboratories
Authors: C.K. Chan, Sandia National Laboratories
L.B. Biedermann, Sandia National Laboratories
A.G. Dylla, The University of Texas at Austin
G.L. Kellogg, Sandia National Laboratories
K.J. Stevenson, The University of Texas at Austin
Correspondent: Click to Email
Nanostructured titania is a promising anode in Li-ion batteries because of its higher operating potential and minimal solid-electrolyte interphase formation. The TiO2(B) polymorph has an open crystal structure, and TiO2(B) nanosheets (NS) have higher surface areas to facilitate faster lithiation kinetics. Structure-dependent lithiation energetics and dynamics have been inferred from rate-dependent cyclic voltammetry, but the underlying mechanisms for this dependence have not been directly measured. We aim to apply low-energy electron microscopy (LEEM) to directly image real-time lithium nucleation and migration on the surfaces of TiO2(B)-NS, and to correlate these behaviors with fundamental chemical and electronic structure properties of the Li/TiO2(B)-NS system. These studies require well-ordered "molecularly flat" TiO2(B)-NS layers, but obtaining these ideal NS assemblies is challenging due to the tendency of nanomaterials to reduce free energy by minimizing surfaces. TiO2(B)-NS generally accomplish this by folding to form nanoclusters. We solved this problem by suspending TiO2(B)-NS in a spreading solvent, depositing the TiO2(B)-NS across a liquid subphase, and transferring them onto substrates by Langmuir-Blodgett (LB) technique. After investigating various spreading solvents and subphases, the best conformal films were obtained using TiO2(B)-NS suspended in TBAOH/water or TBAOH/methanol, and deposited on a water subphase. TiO2(B)-NS were capped by residual ethylene glycol (EG) from their synthesis procedure, and a surfactant like TBAOH was likely required to displace the EG and stabilize the unfolded TiO2(B) sheets. TiO2(B)-NS were deposited by LB technique onto multicrystalline Au substrates. Imaging of these thin conformal films were difficult with standard SEM and AFM techniques, but LEEM showed large areas of densely packed nanosheets with discrete intensity steps. Electron reflectivity spectra was used to identify distinct TiO2(B)-NS regions, where the discrete intensity steps corresponded to discrete increases in the TiO2(B)-NS layer thickness. Low-energy electron diffraction (LEED) patterns of TiO2(B)-NS deposited by TBAOH/water on water indicated rotational disorder in TiO2(B) sheets, with a 1.65 Å-1 diffraction ring consistent only with the (01) spacing of anatase. We believe that TBAOH acts as a surfactant to displace the protective EG capping ligand surrounding the TiO2(B)-NS clusters, allowing the nanosheets to unfold. The resulting water ingress facilitates transformation of the metastable TiO2(B) phase to the thermodynamically stable anatase phase. Preliminary results of in situ surface lithiation of TiO2-NS will also be discussed.