Morphological Dependence of Lithium Insertion in Nanocrystalline TiO2(B) Nanoparticles and Nanosheets

The lithium insertion behavior of nanoparticle (3-D) and nanosheet (2D) architectures of TiO2(B) is quite different, as observed by differential capacity plots derived from galvanostatic charging/discharge experiments. DFT+U calculations show unique lithiation mechanisms for the different nanoarchitectures. For TiO2(B) nanoparticles, A2 sites near equatorial TiO6 octahedra are filled first, followed by A1 sites near axial TiO6 octahedra. No open-channel C site filling is observed in the voltage range studied. Conversely, TiO2(B) nanosheets incrementally fill C sites, followed by A2 and A1. DFT+U calculations suggest that the different lithiation mechanisms are related to the elongated geometry of the nanosheet along the a-axis that reduces Li+−Li+ interactions between C and A2 sites. The calculated lithiation potentials and degree of filling agree qualitatively with the experimentally observed differential capacity plots. SECTION: Energy Conversion and Storage; Energy and Charge Transport T dioxide has attracted significant attention for use as a lithium ion anode material due to its specific capacity, cycling stability at high charge rates, and increased redox potential relative to graphite.1−3 Anatase, rutile, brookite and, most recently, TiO2(B) have been the most widely studied of the eight common polymorphs. Many of these polymorphs have been nanostructured and architecturally modified by means of particle shape control and mesoporous ordering in order to maximize both overall capacity and rate capability. In particular, TiO2(B) exhibits a high specific capacity relative to other titania polymorphs due to its low-density crystal structure. Furthermore, it can be synthesized as nanowires, nanotubes, nanoparticles, and nanosheets, which have been shown to add further capacity.5−8 Recent efforts have focused on understanding how nanostructuring of these metal oxides affects the lithiation behavior in terms of both total storage capacity and rate capability. Here, we present experimental lithiation studies and DFT+U calculations of an ultrathin nanosheet form of TiO2(B), hereafter referred to as TiO2(B)NS. Galvanostatic charge/discharge experiments show unique lithiation behavior for TiO2(B)-NS compared to nanoparticulate TiO2(B) (TiO2(B)-NP). DFT+U Li + site occupancy calculations suggest that a different Li+ intercalation mechanism exists for 2-D nanosheet architectures of TiO2(B) and is related to the elongated nanosheet crystal structure as well as Li+−Li+ repulsive interactions. TiO2(B) nanoparticles and nanosheets were synthesized by a previously reported method described in the Experimental Section. Figure 1a shows TEM of TiO2(B)-NS. The individual sheets range in size from 100 to 200 nm, are highly flexible, and have a tendency to layer on each other. Highresolution TEM (HR-TEM) of TiO2(B)-NS in Figure 1b shows nanocrystalline domains on the surface that are consistent with the (020) lattice spacing of TiO2(B). TEM of TiO2(B)-NP as well as additional images of TiO2(B)-NS are included in the Supporting Information. Figure 2a shows X-ray diffraction (XRD) for TiO2(B)-NS and TiO2(B)-NP that is consistent with bulk TiO2(B) (JCPDS# 741940). The intensity of the (020) peak at 48.6° is significantly higher for TiO2(B)-NS than that for TiO2(B)NP relative to the (110) peak at 25.6°. This suggests that the TiO2(B)-NS lay flat on surfaces, leading to a higher relative intensity of the (020) facet compared to other peaks, which is consistent with previous reports. Raman spectroscopy is a useful tool for characterizing titania phases due to the highly variable local bonding structure of the different polymorphs as well as being able to determine noncrystalline phases that may be unobservable in XRD analysis. The TiO2(B)-NP Raman spectrum in Figure 2b is consistent with that of bulk TiO2(B) (see Supporting Information) and previous reports. The TiO2(B)-NS spectrum is quite similar to both the TiO2(B)-NP and bulk spectra from 200 to 700 cm−1 but deviates significantly below 200 cm−1 as several of the low-energy Ti− O−Ti and O−Ti−O torsional modes are absent at 140 and 150 cm−1. The absence of peaks may be due to dimensional constraint of the 2-D architecture causing these modes to be inaccessible. Through careful synthesis and characterization of the materials, we are confident in the phase purity of the TiO2(B)-NS and -NP materials. TiO2(B) is a kinetic phase of titania, and anatase impurities are often present that can make further interpretation of electrochemical data more difficult. Bruce and co-workers have pioneered much of the work regarding lithiation of TiO2(B) and have shown that nanostructuring significantly increases the specific capacity due to Received: June 13, 2012 Accepted: July 16, 2012 Letter