Observation of lithiation-induced structural variations in TiO₂ nanotube arrays by X-ray absorption fine structure

Abstract

We report here a study of self-organized TiO₂ nanotube arrays both in the amorphous and anatase phases with superior electrochemical performance upon lithiation and delithiation. X-ray absorption fine structure (XAFS) study at the Ti K and L, O K and Li K edges has been conducted to track the behavior. Characteristic features for amorphous and anatase TiO₂ are identified. After lithiation, it is found that although no obvious variation of chemical states is apparent at the Ti K and L edges, charge transfer from Ti 3d to O 2p and also partial amorphization of anatase TiO₂ are evident from spectral intensities. The Li and O K edge XAFS show the successful intercalation of lithium and reveal the existence of a nearly linear “O–Li–O” arrangement in the lithiated TiO₂ nanotube. This study helps in understanding of the lithiation process in nanostructured TiO₂ anodes from a spectroscopic viewpoint.

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Introduction

Nanostructured TiO₂ has been extensively studied and used in photodegradation devices,¹ energy-harvesting systems, such as dye-sensitized photovoltaic cells,^2,3 and energy storage systems such as electrochemical capacitors⁴ and lithium-ion batteries.^5–8 Especially, TiO₂ excites great interest in the battery field due to its high working potential and superior stability, where efforts are being made to expand applications in heavy-duty systems such as electric and hybrid electric vehicles. In order to obtain cells with both high energy density and power density, great attention has been focused on exploring various TiO₂ nanostructures with different phases.^9–12 Furthermore, the geometry or configuration could also significantly affect the performance of TiO₂. Fang¹³et al. and Han¹⁴et al. demonstrate that one-dimensional self-assembled arrays still exhibit excellent electrochemical behaviors at high current and power densities up to 30 A g⁻¹ and 10 C, respectively, which is ascribed to the reduced resistance between the electrode and current collectors and also the improved efficiency of electron transportation along one-dimensional channels.^13–17 Despite the superior electrochemical performance, gleaning a comprehensive understanding of the effect of intercalating lithium ions into TiO₂ frameworks upon alterations of their structures and chemical states, is of paramount significance for both understanding the mechanism and optimization of the engineering design.

Since the report by Whittingham et al.,¹⁸ the insertion of lithium into a TiO₂ framework has been intensively studied in which lithiation was conducted both chemically using n-butyllithium and electrochemically.^7,19–23 Among various characterization techniques such as Raman, NMR,²¹ and XRD,²⁴ X-ray absorption fine structure (XAFS) is arguably the most powerful tool for illustrating the local structural evolution of elements of interest upon lithiation. XAFS probes the modulation of core-level transitions (absorption coefficient) of an element of interest by its chemical environment in both the near-edge and extended regions of the absorption coefficient above the threshold – commonly known as X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), respectively. XANES is an element, local structure and bonding-specific method following the dipole selection rules. EXAFS arises from interference of the outgoing and backscattered photoelectron waves at the absorbing atom and yields information about the local structure of atoms, such as the interatomic distance between the absorbing atom and its neighboring atoms (bond length), coordination number, and Debye–Waller factor (mean square deviation of the bond length) in the first several shells.^25–29

With regard to the structure of lithiated phases of TiO₂ (Li_xTiO₂, 0 < x < 1), extensive work has been conducted; for example, pioneering work from Ohzuku et al.³⁰ claimed the cubic LiTiO₂ phase while Bonino et al.²⁴ and Cava et al.³¹ claimed the orthorhombic Li_0.5TiO₂ phase. Lafont et al.²³ performed an in situ study and found that anatase TiO₂ undergoes a two-stage transition: a Imma orthorhombic Li_xTiO₂ is first formed in a partially lithiated state followed by a cubic LiTiO₂ phase (I4₁/amd) in the fully lithiated state. More importantly, it should be noted that the stability and structure of Li_xTiO₂ are highly dependent on the pristine TiO₂ crystal phase (e.g. TiO₂–B,³² ramsdellite^33,34 and anatase³⁵) and the corresponding morphology (e.g. nanoparticles^19,23 and nanotubes³⁶) of the TiO₂. Borghols et al.³⁷ conducted a Ti K-edge XAFS study on lithiated amorphous TiO₂ nanoparticles, where a pre-edge shift toward lower energy and a new shoulder are found after lithiation which are attributed to the oxygen vacancies. Similarly, Okumura et al.³² also reported the t_2g peak shifted to lower energy in the Ti L₃-edge XANES spectra for submicrosized TiO₂–B. XAFS study of Li_2+xTi₃O₇ ramsdellite³⁸ reveals that though the host lattice remains unchanged upon lithiation, the Ti K-edge EXAFS analysis reveals a slight interatomic distance alteration and a reduced number of unoccupied O 2p densities of states in the O K-edge XANES. Since nanostructured anodes^{7,11,12,21,39,40} exhibit an improvement in the practical capacity approaching theoretical values and also high rate properties, it is crucial to gain additional insights to facilitate both fundamental and practical studies.

In this paper, we set up a protocol for high lithium-ion battery (LIB) performance tracked by XAFS. We first synthesized self-organized TiO₂ nanotube arrays both in amorphous and anatase phases on Ti foil using an electrochemical anodization method and then conducted the lithiation with the electrochemical behaviors evaluated. Finally, a detailed XAFS analysis was conducted to investigate the effect of lithium intercalation on the structure of TiO₂ from the Ti, O and Li perspective.

Experimental methods

TiO₂ NT

TiO₂ NT arrays were synthesized using a one-step anodization process by applying a DC (direct current) power supply with a voltage around 16 V. Ti foil (0.1 mm thick, Goodfellow Ltd.) and a Pt wire were used as anode and cathode, respectively. A glycerol-based electrolyte was used in which HF (0.5 wt%) and glycerol were mixed in a volumetric ratio of 1 : 9. To facilitate the growth of nanotubes with smooth walls, a small amount of H₂O (∼250 μL) was added to the final mixture. Detailed procedures can be found in previously reported results.⁴¹ After 6 hours, the self-organized TiO₂ nanotube was formed aligning vertically onto the Ti foil substrate. Then, the nanotubes were washed several times with deionized water and dried under a N₂ flow. In order to obtain a pure anatase phase,⁴¹ the as-prepared TiO₂ nanotubes were annealed at 550 °C for 2 h, during which TiO₂ was fully crystallized into anatase.

Characterization

Samples were characterized by X-ray diffraction (XRD, Rigaku RU-200BVH with a Co Kα source (λ = 1.7892 Å)), field emission scanning electron microscopy (FE-SEM, Hitachi 4800S), and energy dispersive spectroscopy (EDS). The XAFS experiments were performed at the Canadian Light Source (CLS) located at the University of Saskatchewan. The Ti L_3,2 and O K-edge XANES were measured at the undulator-based spherical grating monochromator (SGM) beamline,^42,43 the Ti K-edge EXAFS were obtained at the soft X-ray microcharacterization beamline (SXRMB),⁴⁴ and the Li K edges were conducted at the variable line spacing plane grating monochromator (VLS-PGM) beamline.⁴⁵ Spectra were normalized to the incident photon flux, which was recorded using a refreshed Au mesh or a Ni mesh.

Electrochemical measurement

The measurements were conducted using two-electrode cells. The as-obtained samples were cut into disks and dried at 80 °C under vacuum overnight to remove the adsorbed water and then directly used as the working electrode considering the good electronic conductivity of Ti foil. Coin cells (CR2032 type) were assembled using lithium metal foil as the counter electrode and polypropylene as the separator inside an argon-filled glove box. The electrolyte was 1 M LiPF₆ in ethylene carbonate/dimethyl carbonate solvent (EC/DMC, 1 : 1 in volume). The profiles of galvanostatic charging and discharging curves were obtained on a computer-controlled battery tester system (Arbin BT-2000) in a voltage range of 1 V to 3 V (vs. Li⁺/Li) with a current density of 100 mA g⁻¹. The specific capacity is calculated based on the mass of TiO₂ NT, which was obtained by stripping the TiO₂ NT from the substrate using adhesive tape. The lithiated TiO₂ nanotubes samples were prepared by discharging the half cells to 1 V at 100 mA g⁻¹ in the first cycle, and then were disassembled from the cell and washed thoroughly with ethanol and DMC.

Results and discussion

The phase composition and crystallinity of the prepared TiO₂ NT arrays are identified by XRD, as shown in Fig. 1a. The as-made TiO₂ NT is amorphous, exhibiting no peaks related to crystalline TiO₂. The three peaks that can be observed come from the Ti foil substrate. The zoomed-in spectrum displays one broad peak centered at 30°, demonstrating its amorphous nature. The EDS spectrum (Fig. S1†) for amorphous TiO₂ clearly shows the Ti and O characteristic fluorescence lines, further indicating the successful preparation of TiO₂. After annealing, typical peaks for tetragonal anatase TiO₂ can be identified at the (101), (004) and (200) planes (JCPDS no. 2-387), indicating a phase transformation to crystalline anatase TiO₂ had taken place. The typical anatase TiO₂ cell is shown in Fig. 1b.

Fig. 1 (a) XRD spectra of the amorphous and anatase TiO₂ NT before and after lithiation, inset is the magnified spectra with angles between 15 and 45 degrees. (b) Scheme of the TiO₂ anatase unit cell. (c) Top view showing the channels along [001] (grey ball: Ti, red ball: O).

The channels along the [001] direction are clearly observable from the top view in Fig. 1c. Intriguingly, for both amorphous and anatase TiO₂, after electrochemical lithiation, no lithium-related new phase can be identified from the XRD spectra, and the original phases are well maintained, indicating that the lithiation process does not change the phase composition markedly. For anatase TiO₂, the (004) plane before lithiation exhibits usually high intensity compared to other planes, illustrating that the well crystallographically oriented TiO₂ NT arrays are along the [001] direction.⁴⁶ It is noted, however, that the intensity of the (004) diffraction decreases drastically after lithiation, indicating a distortion of the TiO₂ plane after the introduction of lithium, which preferentially interacts with TiO₂ along the [001] direction and locates in between (004) planes. It has been reported that the (004) plane has higher surface energy compared to other planes (e.g. the (101) plane) which may facilitate the integration of lithium ions.⁴⁷ Since the lithiation process of TiO₂-based anodes is intercalation in nature, the introduction of lithium into TiO₂ does not significantly alter the integrity of the framework, as shown by the similar intensities and widths of other peaks of the lithiated TiO₂ compared to those of TiO₂ before lithiation.

SEM images of TiO₂ NT arrays both before and after lithiation are shown in Fig. 2. Similar to our previous results, the amorphous (Fig. 2a) and anatase NT (Fig. 2c) grow vertically on the Ti foils with an outer diameter of ∼70 nm. The length is around 400 nm depending on the reaction time.⁴¹ After calcination, the wall thickness increases slightly, leading to a smaller inner diameter for anatase TiO₂ compared with that of amorphous TiO₂. The morphologies of the samples after lithiation are also examined, as shown in Fig. 2b and d. It is found that for both amorphous and anatase TiO₂, the morphologies remain intact after cycling. This observation is in good accord with results reported by Fang et al.¹³ It should be noted that the volume variation upon lithiation for TiO₂ is quite small (around 3%), and that a large surface to volume ratio and the presence of voids in nanotubes help accommodate the stress and strain. As a result, the lithiation process has little effect on the morphology of the aligned nanotubes.

Fig. 2 SEM images of (a) amorphous TiO₂ NT; (b) lithiated amorphous TiO₂ NT; (c) anatase TiO₂ NT; (d) lithiated anatase TiO₂ NT (insets show magnified views of the TiO₂ NTs).

The electrochemical performances of the TiO₂ NT specimens are further examined and the results are shown in Fig. 3. The initial discharge and charge profiles for amorphous and anatase TiO₂ NT are presented in Fig. 3a and b, respectively. From stage A to B, the NTs are discharged and insertion of lithium ions into TiO₂ occurs. In the charge process, the lithium ions are extracted from the TiO₂ framework and stage C is reached. Obviously, amorphous and anatase TiO₂ NT exhibit different voltage plateaus. For anatase TiO₂, it presents discharge and charge plateaus at around 1.7 V and 1.9 V, respectively, corresponding to the insertion and extraction of lithium ions, while the as-made TiO₂, due to the large amount of defects and the disordered structure, shows an expanded potential range. As a result, the curves show no obvious potential plateaus. The shapes of the curves are in good agreement with the earlier reports on amorphous and anatase TiO₂.^12,13,48,49 The amorphous TiO₂ NT delivers an initial discharge capacity of 311 mA h g⁻¹ and a charge capacity of 259 mA h g⁻¹ with a coulombic efficiency of 83%. The anatase TiO₂ NT delivers first discharge and charge capacities of 201 and 180 mA h g⁻¹, respectively. The initial coulombic efficiency is 90%. It is conceivable that the defect-rich and disordered structure in amorphous TiO₂ NT could supply more space for the insertion of lithium ions, leading to a higher capacity compared with that for anatase TiO₂. The cycling behaviors for TiO₂ NTs are shown in Fig. 3c. It is concluded that both amorphous and anatase TiO₂ exhibit quite stable retention of capacity in the first 10 cycles after the initial decay of the capacity, which is due to irreversible solid electrolyte interphase formation.

Fig. 3 Initial charge–discharge profiles of (a) amorphous TiO₂ and (b) anatase TiO₂ at 100 mA g⁻¹ (A, B and C represent different stages for lithiation and delithiation); (c) cycling behavior of amorphous and anatase TiO₂ in the first 10 cycles at 100 mA g⁻¹ in the voltage window of 1.0 V to 3.0 V (hollow dots: discharge curves; solid dots: charge curves); (d) comparison of coulombic efficiency of amorphous TiO₂ and anatase TiO₂ in the first 10 cycles; (e) schematic representation of lithiation process in TiO₂ nanotube arrays.

A higher irreversible capacity in amorphous TiO₂ NT may be due to more trapped lithium ions inside the titanate framework at defect sites^13,50 and a higher amount of solid-electrolyte interface (SEI) layers formed in the thinner tube walls compared to that of anatase. After 10 cycles, the discharge capacities for amorphous TiO₂ and anatase TiO₂ are stabilized at 260 mA h g⁻¹ and 185 mA h g⁻¹, respectively. As shown in Fig. 3c and d, the cycling curves become flat and the coulombic efficiencies of both samples remain around 97% starting from the second cycle, indicating superior cycling performance. The stable cyclability for both NT should be ascribed to the intrinsic small volume variation and also the one-dimensional nanotube morphology, which accommodate the stress and maintain the original structure very well. As demonstrated in Fig. 3e, upon cycling, lithium ions are reversibly intercalated into and deintercalated from the self-organized TiO₂ nanotube arrays, where the structure and morphologies are essentially intact.

Ti L_3,2-edge and O K-edge XANES

To characterize the electronic structure and local structure of the TiO₂ NT before and after lithiation, XANES at the Ti L_3,2 edge and the O K edge have been obtained using partial, element-specific X-ray fluorescence yield monitored with an energy-dispersive silicon drift detector (SDD), which generates a color-coded 2D display of excitation energy versus X-ray fluorescence energy.⁵¹ Fig. S2† shows the 2D display of excitation energy across the Ti L_3,2 and O K edge versus X-ray fluorescence energy recorded by a silicon drift detector (SDD) with the relative intensity color bar as shown in the image. The Ti L_3,2 and O K-edge XANES spectra are obtained by monitoring the response of the corresponding fluorescence (dashed lines) as a function of the excitation energy. There is clearly sufficient energy resolution separating the Ti Lα (452.2 eV) and O Kα (524.9 eV) fluorescence lines.

Fig. 4 shows the Ti L_3,2-edge XANES spectra for amorphous and anatase TiO₂ NT, henceforth denoted AM–TiO₂ and AN–TiO₂ respectively, both before and after lithiation. The features of interest are marked by vertical dashed lines. Typical features arising from the transitions of Ti 2p electrons to previously unoccupied 3d electronic states in a distorted octahedral crystal field can be clearly identified. A weak shoulder doublet in all the compounds, denoted a and b, is related to transitions which are dipole-forbidden, but are possible due to multiple interaction.⁵² Peaks c and f are ascribed to the transition from 2p_3/2 and 2p_1/2 to t_2g, respectively, while d, e and g are attributed to the transition from 2p_1/2 to e_g.^36,41,53,54 It is interesting to note that the relative intensity of d and e is reversed in rutile TiO₂ and in SrTiO₃, where Ti is in a perfect octahedral environment, there is no splitting and d and e merge into a single peak. It is apparent the d, e doublet in Fig. 4a is anatase-like and more pronounced while it is blurred in amorphous TiO₂ NT. Since e_g states are sensitive to the variation in symmetry, the further splitting in spectra at e_g states of the L₃ edge for anatase TiO₂ should be attributed to the distortion to D_2d.^52,54 Since as-made TiO₂ is amorphous and shows a lack of long-range ordering, the local distortion of Ti is an average of all arrangements, leading to the broadening of the e_g peak.

Fig. 4 (a) Ti L_3,2-edge XANES and (b) O K-edge XANES of amorphous TiO₂ NT, anatase TiO₂ NT, lithiated amorphous and anatase TiO₂ NT obtained at stage B in Fig. 3 (AM–TiO₂: amorphous TiO₂; AN–TiO₂: anatase TiO₂).

Turning to the spectra for lithiated samples, it is apparent that the spectra are quite similar to those before lithiation, indicating that the local environment and the electronic structure of Ti are well maintained after the introduction of lithium ions. Since the volume variation for TiO₂ is quite small and it is a tubal structure, the original local structures of the Ti–O framework are well retained after lithiation. Closer observation reveals a broadening feature at the e_g states (d and e) and also the pre-edges (a and b) for the lithiated anatase TiO₂ nanotube, which could be attributed to a slight amorphization or distortion of the crystalline TiO₂ framework by lithium ions as demonstrated from the XRD spectra where the (004) diffraction intensity is greatly reduced. It is conceivable that the lithium ions lie in the [001] direction of the TiO₂ framework along the channels shown in Fig. 1c. These are different from the results reported by Okumura et al.,³² where they claimed a Ti⁴⁺ to Ti³⁺ transition upon lithiation at the Ti L edge in a submicrosized TiO₂–B phase. More importantly, they further compared it with TiO₂–B nanowires of different size.⁵⁵ After lithiation, it is found that the Ti K-edge XAFS spectra of Li_xTiO₂–B nanowires remain unchanged, indicating that the local structural change for Ti is limited in nanoparticles without chemical states variation. Here, for TiO₂ nanotube arrays, no detectable shift of the absorption peaks at the Ti L_3,2 edge is found, indicating that lithium insertion does not lead to a local valence variation of Ti similar to the nanowires reported by Okumura et al.,⁵⁵ instead, it may only change the interaction (e.g. covalency or ionicity) between Ti and O ions analogous to that reported by Zhou et al. previously.³⁶ Also, a slight increase in intensity of the white line is evident for both lithiated amorphous and anatase TiO₂ at the L₃,₂ edge, indicating either electron depletion at the Ti 3d projected orbital, or transition matrix element effects due to the lithium intercalation, or both.

Fig. 4b shows the transitions of O 1s to unoccupied 2p states in the conduction band. The hybridization of Ti 3d and also 4s and 4p with O 2p determines the spectral features. The first two sharp peaks for a and b arise from hybridization with Ti 3d states while the following peaks are ascribed to the O 2p antibonding state and hybridization with s and p states of Ti.^41,53,56 Crystalline anatase TiO₂ shows characteristic features similar to previously reported results,⁵⁷ while amorphous TiO₂ NT exhibits broadened and smoothed-out features, for example, the loss of resonance e due to its low crystallinity and lack of long-range order. However, the locally ordered features could still be tracked, such as peaks a, b and d, similar to those in the Ti L_3,2-edge XANES. The intensity ratio of peaks c to d in the Ti L-edge XANES of AM–TiO₂ is also lower compared with that of anatase, indicating a weaker crystal field or more under-coordinated Ti atoms.⁵⁸

For the lithiated sample, we see that for the amorphous TiO₂ the spectrum matched well with that of TiO₂ before lithiation, indicating a well-maintained local environment for O. The defects and disordered structure in amorphous TiO₂ may help supply extra space to accommodate lithium ions, resulting in little significant influence on the local chemistry for O atoms. However, closer examination indicates a slight increase in the peak intensity ratio of b to a and a slight energy shift of peak a toward lower energy upon lithiation. For the lithiated anatase TiO₂, the peak intensity ratio of b to a increases significantly with a more pronounced feature at peak c and a smoothed-out feature at e. A similar trend, albeit less noticeable, is also found in the amorphous TiO₂ group. A closer look reveals a noticeably weaker resonance at peak a for lithiated anatase TiO₂. Peak a arises from the transition from O 1s to O 2p states which are hybridized with Ti t_2g states, while peak b refers to resonance to 2p states hybridized with Ti e_g states. We also examine the O K-edge XANES of Li₂CO₃, which is a common by-product during charging and discharging in a carbonate-based electrolyte, as shown in Fig. S3.† It exhibits a sharp edge jump at around 539 eV which is assigned to the 1s electrons to the π* (C=O) orbitals, similar to that reported by Yang et al.⁵⁹ Further examination shows no resonances at a (530 eV) or b (532.6 eV) in the O K-edge XANES of Li₂CO₃; thus the suppressed transition at peak a should be exclusively attributed to the lithiated TiO₂, indicating increased occupation of hybrid t_2g states and a charge transfer to 2p states of O character. Since lithium ions are trapped in the crystalline TiO₂ framework, the highly electropositive lithium ions may induce the charge relocation to the O 2p-Ti t_2g bands without affecting the local symmetry significantly. Okumura et al.⁵⁵ also found a similar electron injection into the unoccupied O 2p orbital after lithiation for TiO₂–B nanowires. Furthermore, the enhanced transition at peak c indicates an increment corresponding to the antibonding O 2p transition, which may be caused by the nearly linear O–Li–O arrangement where Li lies in the outgoing path of the O photoelectron wave.^36,60 Finally, the smoothed-out feature e may be due to the lack of long-range ordering and slight amorphization, similar to the lack of splitting at e_g states observed at the Ti L₃ edge.

Li K-edge XANES

Fig. 5a shows the Li K-edge XANES of TiO₂ samples after initial lithiation and 10 cycles. The spectrum for Li₂CO₃ is also provided, which is a common component of a solid-electrolyte interphase in a carbonate-based electrolyte. Lithiated amorphous and anatase TiO₂ nanotubes show similar resonance. The intense transitions at 62 eV and 67.2 eV should be attributable to the multiple scattering of the p wave by the cage environment,⁵¹ while the edge jump at about 60 eV is highly suppressed in all samples, illustrating a more covalent feature for lithium ions and surrounding atoms. Similar to Li K-edge XANES for LiCoO₂ as reported by Zhou et al.,³⁶ where Li ions are also intercalated into the framework, the lithiated TiO₂ NT presents similar resonances with the main edge jump occurring at 62 eV. Upon further comparison of the XANES spectra, it is found that the Li₂CO₃ are formed after cycling. For Li–AN–TiO₂, after the initial lithiation process, the Li₂CO₃ is already formed as can be seen from the shoulder shown in peak e of the spectrum. Since a one-dimensional NT has a large surface to volume ratio, many sites are available for Li₂CO₃ to deposit and coexist with lithiated TiO₂. After 10 cycles of discharge and charge, Li ions are extracted from the TiO₂ framework, while the stable Li₂CO₃ is left behind, thus C–AN–TiO₂ presents distinct resonance features originating from Li₂CO₃, as shown in the first weak shoulder doublet (peaks a and b) and also the edge jump at peaks c and e. A similar trend could also be found in the amorphous TiO₂ group, where more significant features of Li₂CO₃ appeared after 10 cycles. Although it is still unclear what the resonance at 70 eV is, based on the above discussion of Ti L and O K-edge XANES, it most probably arises from the lithiated TiO₂, possibly a multiple scattering between Li and surrounding atoms in the lithiated TiO₂ framework. Fig. 5b shows schematic diagrams of anatase TiO₂ NT upon lithiation. During intercalation, lithium ions locate themselves along the [001] direction forming a nearly linear “O–Li–O” bond in the tubes, while the orientation for highly ordered anatase TiO₂ NT becomes distorted. Thus Li K-edge XANES successfully tracks the intercalation of lithium into the TiO₂ framework and reveals the local structures for Li atoms.

Fig. 5 (a) Li K-edge XANES for lithiated TiO₂ samples at stage B in Fig. 3 (denoted as Li–AM–TiO₂ and Li–AN–TiO₂), the samples after 10 discharge–charge cycles (denoted as C–AM–TiO₂ and C–AN–TiO₂) and standard Li₂CO₃. (b) Schematic diagrams show process of the lithium-driven random orientation for anatase TiO₂ NT. Yellow dots stand for lithium ions which arrange along [001].

Ti K-edge XAFS

Ti K-edge XAFS is also informative about the local structure around Ti atoms, as shown in Fig. 6. The fluorescence yield (FLY) spectrum recorded for the amorphous TiO₂ sample (Fig. S4†) presents a spectrum feature similar to Ti(0) rather than TiO₂. As we know, the penetration depth for X-rays at the Ti K edge is around 20 μm based on calculations from an X-ray calculator;⁶¹ since the TiO₂ arrays have a thickness of 400 nm, thus a FLY spectrum could unveil information coming from the Ti foil underneath. A surface-sensitive total electron yield spectrum is thus used here for TiO₂. Fig. 6a shows the XANES spectra of the pristine and lithiated TiO₂ samples. All the spectra present typical pre-edge features and the white line. For anatase TiO₂, the pre-edge features are marked a, b and c, which arise from hybridization of p and d orbitals of the Ti atom and surrounding neighbors.^62–65 Peak a is attributed to the quadrupole transition of 1s to t_2g states of octahedral TiO₆,^66–69 while peak b and c assignments are controversial – resonance b is either attributed to the 1s to e_g transition of octahedral TiO₆ or the transition to t_2g of neighboring octahedra.^67–69 Peak c is attributed to the transition to e_g states of neighboring Ti atoms or 4p states of absorbing atoms.

Fig. 6 (a) XANES of Ti K edge and (b), (c) Fourier-transformed (FT) Ti K-edge k³-weighted EXAFS spectra for amorphous and anatase TiO₂, both before and after lithiation (k-range for the FT: 2.6–12.8 Å⁻¹). The inset of (a) shows the magnified pre-edge region.

The weak shoulder between c and d stands for the 1s to 4p transition, while peak d is the higher-lying p atomic orbitals.^66,70–73 It is further identified that anatase TiO₂ exhibits well-resolved peaks and sharp features at e and f due to its high crystallinity. In contrast, the amorphous TiO₂ is broader with blurred features. The intense single pre-edge peak suggests Ti-oxide species exist in tetrahedral or pentahedral coordination.^57,74 For poorly crystalline TiO₂, a distorted or defective Ti environment is expected, leading to increased distortion and thus increased resonance intensity at the pre-edge. Similar to Ti L-edge XANES, the spectrum for lithiated samples is nearly the same as the pristine samples, indicating that the TiO₂ framework is well retained after lithiation due to the low volume variation and also the one-dimensional tubal NT structure. The first derivative of XANES spectra shown in Fig. S5† for a lithiated TiO₂ nanotube is almost the same as the pristine ones without an energy shift, further demonstrating that the chemical states are stable for Ti ions upon lithiation.

Fourier transformations (FT) of the EXAFS spectra⁷⁵ are shown in Fig. 6b and c. The FTs for the two phases of TiO₂ exhibit distinct different characteristic features as shown in Fig. S6.† The coordination change can be clearly observed in the EXAFS region (in k-space). For FT-EXAFS in Fig. 6b, A, B, and C stand for the Ti–O, Ti–Ti and Ti–O–Ti coordination shells, respectively.^76,77 The existence of peak C indicates high crystallinity.⁷⁸ For amorphous TiO₂, the Fourier transformation of EXAFS spectra is quite different, while the Ti–O, Ti–Ti and Ti–O–Ti bonds could still be tracked as shown in Fig. 6c. The one major intense peak at shorter interatomic distance, centered at 1 Å, is too short even after a phase correction of ∼0.4 Å to be a real bond distance; it most likely arises from a combination of high k-noise, the isolated Ti-oxide species of amorphous TiO₂ in tetrahedral or pentahedral coordination and the focusing effect of intervening Li in a chemically inhomogeneous Ti local environment. Turning to the spectra for the lithiated samples, we find that both of them exhibit similar EXAFS oscillations in the low k-region compared to pristine samples but with noticeable lower magnitudes and broadening, illustrating a lower coordination number on average of Ti atoms and hence an increase of disorder among them (larger static Debye–Waller factor). It should be noted that a similar feature at position D (around 1 Å in FT) for Li–AN–TiO₂ emerged after the lithiation process, which is similar to that for the amorphous TiO₂. The presence of Li can act as an electron lens modifying the EXAFS with enhanced intensity in the FT. It is concluded that for anatase TiO₂, lithiation of the TiO₂ leads to at least partial amorphization of the pristine TiO₂ which is in agreement with the Ti L-edge XANES.

Conclusions

Amorphous and anatase TiO₂ NT self-organized arrays have been successfully prepared and their lithium-ion storage capabilities are examined. Both of the TiO₂ NTs exhibit stable cycling performances benefiting from the 1D nanostructure and also the inherent low volume variation. Amorphous TiO₂ NTs exhibit higher capacity than anatase TiO₂ NTs due to their higher number of intercalation sites for lithium ions from the disordered structure. A detailed XAFS analysis has been conducted on the pristine and lithiated TiO₂ at the Ti L and K and O K edges. XAFS reveals different local symmetry, distortion and crystallinity in amorphous and anatase TiO₂. Li K-edge XANES confirms the introduction of Li into the TiO₂ framework, locating along the [001] direction of the TiO₂ framework, and its presence induces charge relocation to the O 2p-Ti t_2g bands as demonstrated from the O K-edge XANES. XANES and EXAFS spectra also reveal slight lithiation-induced amorphization of the TiO₂, while the chemical states of Ti and O are well maintained. This work demonstrates a case study of the effects of the lithiation process on structural variations in TiO₂ electrodes probed by the XAFS technique and could be easily extended to other electrodes and energy devices such as photovoltaic cells, capacitors and fuel cells.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: EDS spectrum of amorphous TiO₂ nanotube, 2-D display of excitation energy across the Ti L_3,2 edge and O K edge (y-axis) vs. fluorescence/scattered X-ray energy (x-axis) from Ti and O detected with a silicon drift detector, O K edge XANES of Li₂CO₃ powder, FLY spectrum of amorphous TiO₂ rooted on Ti foil and first-derivative spectra of Ti K-edge XAFS for amorphous and anatase TiO₂, both before and after lithiation. See DOI: 10.1039/c4ta04873c

‡ Current address: Canadian Light Source Inc. 44 Innovation Boulevard, Saskatoon, Saskatchewan, S7N 2V3 Canada.

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