Nanoscale Clarification of the Electronic Structure and Optical

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The Journal of Physical Chemistry

Nanoscale Clarification of the Electronic Structure and Optical Properties of TiO2 Nanowire with An Impurity Phase upon Sodium Intercalation Jun Li,a Zhiqiang Wang,a Ankang Zhao,a Jian Wang,b Yang Song a, c and Tsun-Kong Sham a, c, * a

Department of Chemistry, The University of Western Ontario, Chemistry Building, 1151

Richmond Street, London, Ontario, Canada N6A 5B7 b

Canadian Light Source Inc., University of Saskatchwan, Saskatoon, Canada S7N 2V3

c

Soochow University-Western University Centre for Synchrotron Radiation Research, the

University of Western Ontario, London, Canada N6A 5B7

ABSTRACT

The electronic structure and optical property of heterostructural TiO2 nanowire (NW) synthesized by a hydrothermal process have been extensively studied. X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) were used to probe the crystal structure, and the local structure and bonding (unoccupied electronic states) of the as-obtained TiO2 NW, respectively. Discrepancy between XRD and XANES results without spatial resolution was observed and interpreted on the basis of the inhomogeneous distribution of a sodium titanate

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impurity phase among the “bulk” pristine TiO2 phase. Scanning transmission X-ray microscopy (STXM), on the other hand, provides nanoscale clarification for the divergence with a spatial resolution of 30 nm. Using STXM, the nature of the sodium titanate impurity phase and pristine TiO2 phase was thoroughly characterized by comparing the XANES at the Na K-edge, Ti L3,2edge and O K-edge. The local effect of sodium intercalation into the TiO 2 matrix together with the presence of sodium titanate impurity to the electronic structure of the entire NW specimen was addressed. Analysis was also conducted using 2D XANES−XEOL (X-ray Excited Optical Luminescence) spectroscopy. The optical interplay with structural properties reveals the element and site specificity of the observed luminescence. Of these, the dominant green emission and minor near-infrared (NIR) luminescence were attributed to the defect states from both Ti and O sites within the minor sodium titanate phase and the dominant rutile TiO2 phase, respectively. The relationship between the observed luminescence intensity ratio and the phase content ratio of rutile and sodium titanate is discussed.

INTRODUCTION Owing to its low-cost, non-toxicity and superior photocatalytic activity and stability, TiO2 has been extensively studied as an environmentally friendly photo-catalyst, an additive and an electrode material for optimizing performance of different types of battery.1-5 Recently, monodoping or co-doping with anion/cation species shows a great potential to enhance the electrical conductivity and photoactivity of TiO2 on the basis of band gap modification together with band alignment.1,

6-10

Among these studies, alkali-metal-ion-incorporated titanates attract great

attention in both engineering and science realm. Particularly, lithium titanate, as a “zero-strain” anode material, exhibits long life stability in lithium-ion battery performance.9 Sodium titanate, on the other hand, is reported to be a promising candidate for anode materials in sodium-ion


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battery.8 In addition, using lithium titanate as a substrate which shows excellent sodium storage capability, the optimization of sodium-ion battery performance is achieved.10 In recent decades, hydrothermal method has been widely used for the morphology-tailoring and massive production of one-dimensional (1D) titanate nanostructures.11-12 Of these, alkali titanates can be easily prepared using the alkaline synthesis condition. More importantly, those doped alkali-metal-ions can be easily substituted via ion-exchange.13 Thereby, proton substitution of alkali-metal-ions in titanate nanostructures followed by thermal-annealing is a judicious way for the production of 1D TiO2 nanomaterials in large quantity.14 However, it is an experimental challenge to completely remove all the alkali-metal-ions in the post-reaction process via proton substitution, and the residuals turn out to contribute significantly to the physical and chemical properties of the as-prepared TiO2 nanostructures.13 To understand the structural difference between alkali titanate and TiO2, great efforts have been made to reveal the effect of alkali-metal-ion intercalation into TiO2.11,

13, 15

Despite previous efforts, this

behavior is poorly understood due to the inhomogeneity of the sample and the limitation of traditional X-ray techniques in which the information is often collected on a macroscopic sample with innumerable nanostructures of various size or even composition. Thus a fundamental understanding of the impurity phase dispersion in TiO2 nanostructures and their influence to the electronic structure of TiO2 matrix as a composite is of foremost importance and will pay dividends to the better engineering of this type of multivariate titania nanostructure. Although conventional X-ray techniques, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS), are widely used as the powerful tools to investigate the electronic and crystal structure of nanomaterials in the past several decades, to pinpoint an individual structure in nanoscale is not possible using those


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techniques because they are non-spatially resolved in the nano-domain. Thus only averaged information over an area of typically mm × mm including a large number of individual nanostructures is obtained. Therefore, it is difficult to extract the information of the component of interest such as an impurity phase from the “bulk” sample if it is in low quantity, or the element of interest exits in both the impurities and the “bulk” sample. The afore-mentioned challenges can be overcome with scanning transmission X-ray microscopy (STXM), which has been extensively developed using undulator based thirdgeneration synchrotron light sources with highly collimated and polarization-controlled beams, providing an unique capability to probe the electronic structure of isolated nanostructures with a nanoscale spatial resolution.16 STXM is the state-of-the-art methodology for studies on the electronic and structural properties of hetero-nanostructures. It not only possesses the highspatial resolution in nanoscale but also provides the unique capability for chemical speciation in an individual nanostructure; that is, spectro-microscopy, a combination of XAS and X-ray microscopy, with a spectral and spatial resolution of 0.05 eV and 30 nm, respectively.17-18 Successful investigations of the electronic structure and surface defects of individual (hetero-) nanostructure using STXM make it an extremely desirable technique in modern nanoresearch.1723

Two-dimensional X-ray absorption near edge structure−X-ray excited optical luminescence (2D XANES−XEOL) spectroscopy, an advanced spectroscopic technique to study the interplay of structural and optical properties of nanostructures, has also been used in this study.24-26 XANES reveals the local symmetry and unoccupied electronic states associated with the absorbing atom.27-28 XEOL, on the other hand, is a de-excitation spectroscopy which measures the radiative recombination of thermalized electron−hole pairs in the solid upon core-electron


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excitation.27, 29 XEOL can be used to track the optical band gap and energy transfer to defect states in the nanostructure, especially semiconductor and metal oxides. The 2D XANES−XEOL spectroscopy is accomplished by simultaneous measurement of XANES and XEOL; that is that XEOL spectra in the full optical range of 200 nm−960 nm are collected with a CCD detector when the excitation energy is tuned across the edge of interest, an energy step at a time, in which the variation of XEOL intensity (in wavelength selected or in total yield) in turn can be recorded as photoluminescence yield (PLY) or optical XANES. Therefore, optical interplay with structural information is recorded in a 2D display with the intensity color coded. Due to element specific core-electron excitation, 2D XANES−XEOL is capable of providing information on the element and the site responsible for the observed luminescence of testing structure via a thermalization process. More details of the 2D XANES−XEOL technique is reported elsewhere.24-26 In this study, TiO2 nanowires (NWs) synthesized by a hydrothermal method in aqueous NaOH are the particular samples of interest. Considerable works have been done to focus on the ideal conditions for certain types of 1D TiO2 nanostructures fabrication in terms of synthetic temperature, precursor types and alkaline concentration.13,

30-31

However, post-annealing

treatment of those nanomaterials is also very interesting due to the complexity of the morphology, phase and structure transformation induced by annealing.15 Herein, we report the electronic structure and optical property investigation of multivariate titania nanostructures, including sodium titanate and pristine TiO2 using XRD, XANES, STXM and 2D XANES−XEOL spectroscopy. The former two are compared to clarify the effect of sodium titanate impurity phase on the electronic structure of the entire nanostructure sample. The nature of sodium titanate and rutile TiO2 is thoroughly investigated by the XANES analysis at the Na


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K-edge, Ti L3,2-edge and O K-edge using STXM. 2D XANES−XEOL spectroscopy is used to study the correlation between structural and optical properties of sodium titanate and rutile TiO2. EXPERIMENTAL SECTION Sample Preparation TiO2 NWs were prepared using the hydrothermal method.11, 13 Typically, 1 g of anatase TiO2 nanoparticles with an average grain size of ~25 nm (Nanostructured & Amorphous Material Inc.) together with 40 mL of NaOH (10 M) aqueous solution were put into a 60 mL Teflon-lined bottle. Then the Teflon bottle was transferred to a stainless steel autoclave, sealed, and placed in the oven at 200 ℃ for 24 hours without any shaking or stirring. After the autoclave was naturally cooled to room temperature, the as-synthesized sample (white precipitate at the bottom) was sequentially washed with 1 wt % HCl aqueous solution for Na+ ion substitution by H+ ion, deionized water and anhydrous ethanol for further sample purification until pH = ~7. Then the sample was dried at 70 ℃ for 6 hours in air to obtain the white soft fibrous powder. The asprepared sample was divided into two parts for further thermal-annealing at 900 ℃ and 1000 ℃ under ambient condition to initiate crystallization and anatase-to-rutile phase transformation, henceforth denoted as NW900 and NW1000, respectively. For STXM sample preparation, 10 mg of NW1000 powder was ultrasonically dispersed in ethanol, and a few drops of the as-prepared solution were loaded on a holey TEM grid (400 mesh), which was used for both TEM and STXM characterization. Characterization The crystal structure and morphology of the NWs sample were characterized by XRD (Rigaku RU-200BVH) with Co Kα radiation (λ = 1.7892 Å), scanning electron microscopy (SEM, LEO


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1540XB) and transmission electron microscopy (TEM, Philips CM10). Energy dispersive X-ray (EDX) spectroscopy was recorded using an EDX detector (attached to SEM facility). Synchrotron measurements were conducted at the Canadian Light Source (CLS). Ti L3,2-edge and O K-edge XANES using both total electron yield (TEY) and fluorescence yield (FLY) modes were obtained on the Spherical Grating Monochromator (SGM) beamline with high resolution E/∆E > 5000.32 TEY was detected with the specimen current whereas X-ray fluorescence photons from element of interest were collected by four silicon drift detectors (SDD) contributing to FLY. Since fluorescence X-ray has a much longer escape depth than electron, FLY is bulk-sensitive whereas TEY is surface-sensitive. 2D XANES−XEOL map was generated by simultaneously recording the XEOL at the full wavelength range (200 nm − 960 nm) using a dispersive spectrometer (QE65000, Ocean Optics) while tuning the excitation energy across the Ti L3,2-edge and O K-edge in small energy steps. The photoluminescence yield (PLY) XANES at the total (zero order, 200 − 960 nm) and wavelength-selected regions (green: 400 − 650 nm and near-infrared (NIR): 750 − 900 nm) were obtained from the map with selected energy windows. The NW1000 specimen was mounted on a carbon tape attached to a sample holder with an angle of 45°toward the photon beam. All XANES spectra were normalized to the incident photon flux. STXM measurement was performed at the soft X-ray Spectromicroscopy (SM) beamline. The design and working principles of STXM are reported elsewhere.33 Generally, the monochromatic X-ray beam was focused on the sample using a 25 nm outermost-zone zone plate (CXRO, Berkeley Lab) with a 30 nm diffraction-limited spatial resolution. The same sample region of interest (ROI) of the NW1000 sample was used to record the image sequence (stack) scans over


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different photon energy ranges at the Na K-edge, Ti L3,2-edge and O K-edge. Details of STXM measurement and data analysis are included in the Supporting Information. RESULTS AND DISCUSSION

Figure 1. XRD pattern of NW900 (black), NW1000 (red), Na2Ti6O13 (orange, JCPDS 37-0951), anatase (blue, JCPDS 21-1272) and rutile (magenta, JCPDS 21-1276). The XRD results of TiO2 NWs after annealing at 900 ℃ and 1000 ℃ are shown in Figure 1. The NW900 shows mostly anatase phase with some weak diffraction peaks corresponding to Na2Ti6O13, whereas the XRD pattern of the NWs after annealing at 1000 ℃ exhibits a dominant rutile phase as well as a minor Na2Ti6O13 phase. This finding is consistent with a previous report of a comparable phase evolution study of sodium titanate by Sun and Li13 where the assynthesized nanostructure via hydrothermal reaction in NaOH can transform TiO2 to Na2Ti9O19 at 600 ℃ followed by a multiphase with Na2Ti6O13 and pristine TiO2 after annealing at 850 ℃. These results indicate that inhomogeneous distribution of Na+ ions and phase separation take place during structural evolution. They also claimed that Na+ ions are strongly correlated to the thermal stability of the titanate nanostructure thus prepared. In our case, TiO2 NWs with a slight amount of Na+ ions can retain the anatase phase with an annealing temperature as high as 900 ℃ whereas a suddenly exhaustive structure transformation to rutile phase is produced after annealing at 1000 ℃ although the impurity Na2Ti6O13 phase persists.


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Fine structures at the Ti L3,2-edge XANES of both NW900 and NW1000 in comparison with standard anatase and rutile (Sigma-Aldrich) are shown in Figure 2a. Clearly, both NW900 and NW1000 show the general spectral features of the distorted TiO6 octahedral structure by sharing the similar XANES patterns with standard anatase and rutile:34-35 p1 and p2 at the pre-edge are attributed to core hole-d electron coupling; peaks a and b at the Ti L3-edge are assigned to dipole excitations from Ti 2p3/2 states to t2g and eg unoccupied states, respectively; features c and d at the Ti L2-edge correspond to electronic transitions from Ti 2p1/2 states to t2g and eg unoccupied states, respectively.

Figure 2. Ti L3,2-edge (a) and O K-edge (b) XANES of NW900 and NW1000 in comparison with commercial anatase and rutile powder samples recorded in total electron yield (TEY) mode. The evidence for the local distortions from Oh symmetry to D2d and D2h symmetries for anatase and rutile, respectively, is provided by the peak splitting of b to b1 and b2. Whereas anatase shows a more intense feature b1 compared to b2, rutile is the reverse case. These spectral characteristics have been well established by experiment and theory.26, 34-35 As shown in Figure 2a, the intensity ratio Ib1/Ib2 comparison between synthesized and standard samples suggests that the local structures around the titanium atoms are slightly different. For NW900, Ib1/Ib2 > 1


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indicates the anatase-like structure of NWs after annealing at 900 ℃, which is consistent with XRD result including the dominant anatase phase. However, nearly equivalent intensity of peak b1 and b2 is observed for NW1000. By simply considering the dominant pure TiO2 phase, our very recent work focusing on crystal phase evolution of TiO2 hierarchical nanostructure via tracking Ti L-edge XANES showed that a phase transformation from meta-stable anatase to thermal-stable rutile can be induced by increasing annealing temperature. An intermediate multiphase (anatase-rutile mixture) structure can be achieved once the annealing temperature is set to the phase transition temperature window, resulting in the variation of Ib1/Ib2 value between anatase and rutile,26 i.e., (Ib1/Ib2)rutile < (Ib1/Ib2)multiphase < (Ib1/Ib2)anatase. Thus, regardless of the XRD result of NW1000, the fine structure of NW1000 might, at first glance, indicate its multiphase nature with similar amount of anatase and rutile. Consistent results from the bulk of NW900 and NW1000 recorded as FLY XANES at the Ti L3,2-edge are shown in Figure S1a although the general FLY spectra get damped due to self-absorption.27 A discrepancy arises here because the XRD spectrum of NW1000 only shows dominant rutile phase without the noticeable presence of anatase. Although the presence of some weak diffraction peaks hints the existence of Na2Ti6O13 impurity phase, no direct evidence shows that such a small amount of sodium titanate impurity phase can result in the perplexed electronic structure of rutile-dominant TiO2 nanostructure. The influence of Na2Ti6O13 involvement to the electronic structure of NW900 and NW1000 can be further confirmed by XANES analysis at the O K-edge (Figure 2b):28 peak A and B are attributed to electronic transitions from O 1s to O 2p covalently hybridized with t2g and eg states of Ti 3d band, respectively, whereas peak C and D are assigned to electronic transitions from O 1s to O 2p covalently hybridized with Ti 4sp states, and the presence of peak E is an indicator of


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the long range order of titanate materials. The blue shifts of feature C and E together with the peak-splitting of D into D1 and D2, compared to anatase (D2d local symmetry), suggest the D2h local symmetry of rutile. Thus at first glance, NW900 displays an anatase-like XANES feature whereas NW1000 shows a rutile-like local structure by a simple comparison with standard samples. However, the red-shifted feature D of NW900 and the unresolved feature D2 of NW1000 compared to standard anatase and rutile, respectively, further demonstrates that despite the similarity, there exists noticeable inconsistence of the electronic structures between synthesized and standard samples. Consistent results from the bulk of NW900 and NW1000 recorded as FLY XANES at the O K-edge are shown in Figure S1b.


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Figure 3. TEM image (a) of a branching NW1000 and its averaged STXM optical density image (b) from all stack images at the Na K-edge, Ti L3,2-edge and O K-edge. The region of interest for STXM measurement shown in (b) is indicated by the grey rectangular region in (a). (c) Thickness distribution of ROIs corresponds to their Ti L3,2-edge XANES absorption edge jumps in (d), respectively. Ti L3,2-edge (d) and O K-edge (e) XANES spectra extracted from different ROIs (1~8, indicated in Figure 3b) together with an average spectrum (sum) of these ROIs in


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comparison with the TEY and FLY spectra of standard anatase and rutile. The scale bars of (a) and (b) are 2 μm and 1 μm, respectively. In general, these discrepancies could not be well resolved due to the somewhat ambiguous nature of the sodium titanate phase and the complexity of the mixture of the Na2Ti6O13 and the pristine TiO2 phase. To this end, STXM provides its uniquespectro-microscopic capability to investigate the electronic structure in nanoscale for clarification. In this study, NW1000 is primarily taken as the sample of interest for STXM characterization due to its more intriguing spectral feature as shown in the above XANES results. The hierarchical morphology of NW1000 is clearly observed using TEM (Figure 3a) and SEM (Figure S2). The branching nanostructure indicates the gain of grain size at the expense of neighboring NWs during high temperature annealing. EDX spectroscopy (Figure S3) shows the presence of Ti, O, Na and C in the NW1000. Generally speaking, both TiO2 (major component) and Na2Ti6O13 (impurities) contribute to the strong signals of Ti and O, whereas the weak Na signal comes from the impurity phase. The weak C signal is from the carbon tape on which the sample was placed for the SEM and EDX characterization. To reveal the distribution of the impurities in the TiO2 NWs, detailed characterization of the morphology and electronic structure of the branching NW1000 sample was carried out using STXM. Figure 3b shows its averaged STXM optical density (also called absorbance which is related to the absolute thickness of the sampled region, see supporting information) image from all stack images at the Na K-edge, Ti L3,2-edge and O K-edge with the sample of interest marked by a grey rectangular in Figure 3a. The Ti L3,2-edge XANES spectra of various ROIs obtained in transmission mode and an average (sum) spectrum of these ROIs in comparison with standard anatase and rutile are shown


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in Figure 3d. Generally speaking, all ROIs show similar spectral pattern as standard samples, suggesting they have similar TiO6 local structure. Several interesting observations can be noted with a further analysis. First, unlike the well-resolved fine structures (peak a ~ d) of standard samples as indicated by the TEY mode, the Ti L3,2-edge XANES of almost all ROIs obtained by STXM present profiles similar to the FLY of the standard samples. Of these, the resolvability of the fine structures decreases with the damped spectra. Second, fine structure comparison among ROIs indicates that two different types of structures are presented. Spectra from ROI-1 to ROI-5 show the rutile FLY XANES whereas ROI-6 to ROI-8 display the spectra pattern inconsistent with either anatase or rutile, suggesting the presence of another unknown phase in this branching NW1000. Structural discrepancy among ROIs can be further confirmed by investigating their O K-edge as shown in Figure 3e, in which TEY and FLY spectra of standard samples are included for comparison. Consistent with the results from the Ti L3,2-edge XANES of ROIs, the O K-edge spectra from ROI-1 to ROI-5 clearly show the rutile characters. However, ROI-6 to ROI-8 display the O K-edge XANES profiles similar to the anatase phase. But the increasing intensity ratio of IA/IB together with the red-shifted feature D of the unknown phase relative to the anatase phase still suggests their different local structures. Thus, XANES results under nanoscale characterization from both Ti L3,2-edge and O K-edge XANES demonstrate the multiphase nature of this branching NW1000 which contains spectral feature of rutile TiO2 and another unknown phase. We now return to the sodium titanate impurity phase in NW1000 as revealed in XRD. Literature on the electronic structure of sodium titanate nanostructures36-37 echoes the fine structures of the unknown phase (ROI-6, 7, 8) of NW1000. Besides, the Na K-edge XANES


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extracted from various ROIs (Figure S4) show that only ROIs of the unknown phase contain the Na K-edge character at 1080.4 eV corresponding to the Na 1s to 3p transition, whereas Na signal from the other ROIs related to rutile phase is barely observed. Thus the involvement of sodium titanate (Na2Ti6O13 from XRD) and rutile TiO2 are mainly considered and discussed below. First, let us consider the damping in XANES obtained with STXM, especially at the Ti L3,2edge. It is caused primarily by the thickness effect of the sample.26-27 Given the different density between sodium titanate (Na2Ti6O13, density = 1.76 g cm-3) and rutile TiO2 (density = 4.25 g cm3

), thickness distribution calculations are performed for sodium titanate ROIs (6 ~ 8) and rutile

ROIs (1 ~ 5) based on the thickness modelling (Supporting Information). The resulting thickness distribution corresponding to the edge jump value (vertical intensity difference between postand pre-edges) of each ROI is shown in Figure 3c. Clearly, the thickness of rutile and sodium titanate ROIs are comparably larger and smaller than their one absorption length (Figure S5), respectively, in transmission measurements recorded in STXM, which illustrates the better resolved spectral features of Na2Ti6O13 than rutile. The negligible thickness effect at the O K-edge in FLY and STXM is mainly due to the higher energy of the O Kα X-rays than that of the Ti Lα X-rays, suggesting the larger escape depth of the former than the latter. Second, as shown in Figure 3d, spectral, hence structural difference at the Ti L3,2-edge between sodium titanate and rutile is mainly from the eg band at the L3-edge (feature b). The splitting of the twofold peak b into a shoulder feature b1 and an intense peak b2 is an indication of the D2h local symmetry of rutile structure as discussed above. Sodium titanate, on the other hand, only shows an asymmetric feature b without splitting. Guttmann et al.36 claimed that the absence of obvious Ti L3-eg band splitting in (Na,H)TiO2 nanoribbons is due to an averaging


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effect over multiple types of titanium octahedral states in the nanoribbon, i.e., that it is related to the multiple stacking of a distorted TiO6 octahedral structure. Results provided by Andrusenko et al.38 show that the structure of resulting sodium titanate is constructed by the intercalation of Na+ to the rigid layers of titanate octahedra with the help of the relatively weak Coulomb interactions (Figure S6). Thus the dislocation of these weakly bound species can cause lattice shift in the layers very easily, especially those locating near the structure edges, resulting in an interruption of the long-range order of TiO2 and thus the reduction of eg band splitting of sodium titanate at the Ti L3-edge. Interestingly, with the inclusion of both sodium titanate phase and rutile phase, the average eg-band (average spectrum in Figure 3d) presents an almost equivalent intensity of features b1 and b2, which mimics the spectrum pattern of NW1000 as shown in Figure 2a. Therefore, the discrepancy addressed by comparison of the XRD result and previous XANES analysis in Figure 2a can be well interpreted with the help of nanoscale clarification using STXM: the equivalent intensity of features b1 and b2 of NW1000 in Figure 2a is not due to anatase-rutile mixture but the average of onefold and twofold feature b of sodium titanate and rutile TiO2, respectively. The unnoticeable difference observed at the Ti L3-t2g band (feature a) between sodium titanate and rutile is due to the weak interaction between t2g orbitals (dxy, dxz and dyz) and O 2p orbitals. In contrast, the high sensitivity of the Ti L3-eg band, which benefits from strong directional bond formation between eg orbitals (dx2-y2 and dz2) and O 2p orbitals towards the oxygen ligands, can be used to differentiate structures with different local symmetry around the metal cations.37, 39 In order to reveal the structural difference from the O K-edge XANES between sodium titanate and pristine TiO2, Bittencourt et al.37 calculated the contribution of oxygen atoms at different sites to the O K-edge XANES spectrum based on their sodium titanate model using the local


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density approximation. They claimed that the intensity of peak B (or the A/B intensity ratio) is correlated with the O-Ti coordination: the higher the O-Ti coordination, the more intense the peak B (i.e., the A/B intensity ratio decreases). The red-shifted peak D in sodium titanate corresponding to the hybridization of O 2p with delocalized states (Ti 4sp bands) is due to the structural changes around the O sites induced by the intercalation of Na+ ions to the titania system. Therefore, nanoscale clarification of the electronic structure of NW1000 using STXM shows a mixture of sodium titanate phase and rutile phase. Compared to the absorption fine structures of standard anatase and rutile, the XANES analysis of the anatase-like structure of sodium titanate indicates the decreasing of O-Ti coordination and further local and non-local (long-range order scale) distortion around the Ti and O sites by the intercalation of Na+ ions into the anatase TiO2 matrix. Consistent results can be found in previous XAS study of sodium titanate40 and the crystal structure comparison between sodium titanate and anatase37 where different types of oxygen and titanium sites contribute to sodium titanate XAS whereas all oxygen and titanium atoms are structurally equivalent in anatase TiO2 (same for rutile TiO2). So the Ti L3,2-edge and O K-edge XANES spectra of sodium titanate are an average over these different titanium and oxygen local sites. Using the Ti L3,2-edge reference spectra of sodium titanate and rutile TiO2 for thickness modeling and chemical imaging (Supporting Information), the chemical maps of sodium titanate and pristine TiO2 are shown in Figure 4a and 4b, respectively, and their absolute thickness distribution is indicated by the corresponding grey scales in nanometers. Clearly, only the far left branch contains the sodium titanate structure, whereas the majority shows the structure of pristine rutile TiO2. Further evidence is provided by STXM chemical maps of Na, Ti and O


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elements as shown in Figure 4c, 4d and 4e, respectively, in which the red and blue regions represent the imaging of sodium titanate and rutile TiO2, respectively.

Figure 4. STXM chemical maps of (a) sodium titanate and (b) rutile TiO2 as well as their corresponding thickness distribution in nanometer indicated by their right-side grey scale. STXM chemical maps of (c) Na, (d) Ti and (e) O elements in sodium titanate (red) and rutile TiO 2 (blue). The scale bars in all STXM images are 1 μm. The optical properties of titanate nanomaterials are not well understood to date. To fully understand the structural interplay with optical properties, 2D XANES−XEOL spectroscopy is used to study the multiphase NW1000. Since the X-ray absorption threshold of O 1s core electron is higher than that of the Ti 2p electrons, the Ti 2p core electrons are also excited at the O K-edge. 2D XANES−XEOL mapping of the NW1000 with photon energies scanning across the O K-edge is shown in Figure 5a. At first glance, NW1000 displays one broad green emission band at ~500 nm (~2.5 eV) and a weak NIR peak at ~820 nm (~1.5 eV). The XEOL intensity of the NIR band is severely suppressed once the excitation energy is tuned on the O K-edge.


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Figure 5. (a) 2D XANES−XEOL map of the NW1000 with excitations across the O K-edge (520 − 560 eV), in which the x-axis is recorded as wavelength in nm whereas the y-axis stands for excitation energy in eV. XEOL intensity is indicated by color bar on the right-side. (b) Energyselective XEOL spectra across the O K-edge taken by a series of horizontal cuts of the 2D XANES−XEOL map (excitation energies of varies XEOL spectra are indicated using colorcoded arrows in (c), in which excitation energy point at 520 eV is not shown). (c) Wavelengthselective PLY spectra taken by vertical cuts of 2D XANES−XEOL map in comparison with O K-edge XANES of Na2Ti6O13 and rutile TiO2 (taken from ROI-6 and ROI-5 in Figure 3e, respectively). PLY spectra at the green and NIR regions are integrated from 400 nm to 650 nm and from 750 nm to 900 nm, respectively. The zero-ordered PLY spectrum is the result of integration of the whole wavelength region (200 − 960 nm).


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A series of energy-selective XEOL spectra across the O K-edge taken by horizontal cuts of the 2D XANES−XEOL map are shown in Figure 5b. Clearly, with a multiphase nature of minor sodium titanate and major rutile TiO2, NW1000 displays the major green emission band as well as the minor NIR luminescence. When the excitation energy is tuned across the O K-edge, the intensity variations of these two bands behave in a totally different way: the XEOL intensity of green band is slightly increased whereas the NIR luminescence is almost totally quenched once the excitation energy is above the O 1s absorption threshold, suggesting their different luminescent mechanisms. Wavelength-selective PLY spectra (taken by vertical cuts of 2D XANES−XEOL map) in comparison with O K-edge XANES of Na2Ti6O13 and rutile TiO2, are used to understand the optical interplay with the structure of NW1000. As shown in Figure 5c, O K-edge XANES recorded as PLY in the green region (400 − 650 nm), NIR region (750 − 900 nm) and full region (200 − 960 nm, zero order) are compared with the O K-edge XANES of Na2Ti6O13 and rutile extracted from ROI-6 and ROI-5 (Figure 3b), respectively. First, it is worth mentioning that PLY, unlike TEY and FLY which reflect the averaged fine structures, is more structure and morphology sensitive and provides the fine structures of luminescent materials at certain optical channel. Indeed, PLY at the green region bears some resemblance of the XANES profile of Na2Ti6O13, especially the fine structures above the O Kedge as indicated by the orange dotted lines. Meanwhile, PLY in the NIR region shows the more rutile-like fine structures although the entire PLY spectrum is totally inverted. Further evidence is provided by PLY analysis at the Ti L3,2-edge shown in Figure S7. Thus the major green and minor NIR luminescence can be assigned to the defect states of the minor sodium titanate and major rutile TiO2, respectively, as the emission energies (~2.5 eV for green and ~1.5 eV for NIR)


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are lower than the band gaps of sodium titanate (~3.3 eV)41-42 and rutile (~3.03 eV).43 It has been noted in previous luminescence studies of sodium titanate44 and rutile26-27 structures that the former phase emits green luminescence whereas the latter phase shows the NIR emission band. In this case, both titanium and oxygen defects are responsible for the two luminescent bands as their optical XANES (PLY) spectra are highly correlated with their corresponding STXM XANES spectra. Second, the inverted PLY XANES of NWs, in this case, suggests that the excitation does not couple to the optical channel effectively. That is that once the excitation energy reaches the edge jump, the number of optical photons produced per excitation photon absorbed becomes less efficient compared to absorption below the edge while the total number of photons absorbed are similar under total absorption condition;45 meanwhile, the concomitant abrupt decrease of attenuation length accords with the sharp increase of X-ray absorption at the absorption edge, a fraction of the Auger electrons that were turned on by the creation of the O 1s core hole will escape from the NW surface without contributing to the optical luminescence via energy transfer (incomplete thermalization); below the edge the penetration depth of the photon is much deeper and the corresponding energy of the photoelectrons have a better chance to undergo complete thermalization and transfers the energy to the bulk based optical channel more effectively. The denser rutile structure (density = 4.25 g cm-3) makes it suffer more of the above situation than the sodium titanate phase (density = 1.76 g cm-3) as the attenuation length of the former phase decreases more significantly than the latter phase (Figure S5), resulting in the totally inverse PLY at the NIR band (rutile character) and partial inverted PLY at the green region (sodium titanate character).


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In addition, the relative intensity between the green and NIR emissions is also interesting. As shown in Figure 5b, the XEOL intensity ratio of green comparing to NIR emission is not proportional to the phase content ratio of sodium titanate and rutile. Because as the major component of NW1000, the rutile TiO2 emits the minor NIR emission band which is highly suppressed by the presence of sodium titanate impurity phase. On one hand, it may be due to the efficient inter-phase energy transfer from rutile phase to sodium titanate phase. Considering the multiphase nature of NW1000, energy transfers are competing from absorbed photons to the green and NIR optical channels. In this case, a mixed phase of rutile and sodium titanate may result in the unique trap states modifying the inter-phase carrier transport and thus enabling the charge carriers to recombine with defects to generate green emission. One example of this type of energy transfer in hetero-nanostructure can be found in a previous photoluminescence study of commercial P25 (anatase and rutile multiphase structure) which shows strong visible emission whereas no rutile emission at NIR region is detected despite the 20 % rutile content.46 On the other hand, given the fact that the sodium titanate and rutile TiO2 phases are largely spatially separated under nanoscale clarification. The non-intimate contact between those two components could hardly harvest such an efficient inter-phase energy transfer as discussed above. So another more reliable explanation is the formation of highly crystalline rutile TiO2 after annealing at 1000 ℃. As high temperature annealing is well-known for improving the crystallinity of nanostructures,47-48 the rutile phase fabricated in this condition has the fine crystalline structure as confirmed by the XRD (Figure 1) result where the narrow diffraction line shape with welldefined rutile pattern is presented, thus the defects responsible for the NIR luminescence are mostly eliminated. Therefore, the discrepancy between phase contents and their corresponding


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XEOL intensities mainly results from their crystallinity difference (high defect concentration and high emission efficiency in sodium titanate). CONCLUSIONS XRD, XANES, STXM and 2D XANES−XEOL spectroscopy have been applied to investigate the electronic structure and optical property of TiO2 NW prepared by a hydrothermal process and subsequent proton substitution followed by thermal-annealing. XRD results show that the crystal structure of the NW is mainly anatase after annealing at 900 ℃ whereas an exhaustive phase transformation to rutile is obtained after annealed at 1000 ℃ although a slight amount of Na2Ti6O13 impurity phase is also present. XANES characterizations of NW900 and NW1000 at the Ti L3,2-edge and O K-edge present some perplexed features which are not fully aligned with standard anatase and rutile TiO2. Nanoscale clarification of heterostructural NW1000 using STXM well illustrates the presence of both pristine TiO2 phase and sodium titanate impurity phase. The major differences between these two components are the Ti L3-eg band splitting and the intensity ratio of the two pre-edge structures at the O K-edge, indicating the decreasing of OTi coordination and further local and non-local distortions around the Ti and O sites of sodium titanate by the intercalation of Na+ ions into the anatase TiO2 matrix. Consequently, the average over different titanium and oxygen sites in sodium titanate contributes to the local structures of Ti and O, respectively, in the XANES. Optical interplay with structural properties of NW1000 is analyzed using 2D XANES−XEOL spectroscopy. The good alignment between optical XANES (PLY) and STXM XANES reveals the origins of the observed green and NIR luminescence where the sodium titanate impurity phase contributes to the dominant green emission band whereas the major rutile phase emits the minor NIR band. The discrepancy between phase content ratio and luminescence intensity ratio is interpreted by structural clarification in


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nanoscale: the observed minor NIR emission from dominant rutile NW is mainly attributed to the well-crystallized rutile phase after high temperature annealing, resulting in the healing of luminescent defects and thus quenching the NIR luminescence.

ASSOCIATED CONTENT Supporting Information Figures S1 – S7 and additional text. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Research at the University of Western Ontario is supported by NSERC, CFI, OIT and CRC (TKS). Synchrotron work done at CLS is supported by NSERC, CFI, CHIR, NRC and the University of Saskatchewan. J.L. acknowledges the receipt of support from the CLS Graduate Student Travel Support Program.We would like to thank the SGM beamline scientists Dr. Tom Regier for his technical support at CLS.


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