3. Phase transformation and luminescence from TiO2 nanotubes

TiO2 NT made by anodization has an amorphous structure. Upon annealing, they crystalize into anatase structure, and when temperature increases further, phase transformation occurs and anatase gradually turns into rutile. The morphology of the nanotube retains at the amorphous and anatase phase, but entirely collapses when rutile phase dominates. Figure 6 shows the morphology evolution of TiO2 NT undergoing phase transformation [37]. The tubular structure gradually collapses and fused into bulky columns. The phase transformation in nanostructured TiO2 has been reasonably well studied. Although the exact temperature for phase transformation to occur depends on material morphology, the anatase to rutile transformation is the common outcome when the calcination temperature increases.

Figure 6. Scanning electron microscopy (SEM) images of TiO2NT at different calcination temperatures. (a) as-prepared (inset is the cross section), (b) 500C, (c) 650C, and (d) 800C. (Adapted from Ref. [37]).

Figure 7a shows the evolution of XANES of TiO2 NT as calcinated temperature increases. The as-prepared TiO2 NT exhibits broad features due to its amorphous form. The eg peak starts to show splitting at calcination temperature above 400C indicating the formation of anatase phase, and the more intense b1 than b2 in the eg splitting corresponds to anatase features. As the temperature increases, the b doublet persists, but the b1/b2 intensity varies. At 650C, b1 and b2 exhibit similar intensities, indicating a mixed phase of roughly equal concentration. Higher temperature calcination produces a pure rutile phase. A linear combination fit can be conducted using XANES of anatase and rutile standards (e.g. Figure 2), as shown in Figure 7b.

vertical cut is a typical XANES spectrum, and the horizontal cut is a XEOL spectrum. In this way, both the change of absorption coefficient as well as the luminescence as a function of

TiO2 NT made by anodization has an amorphous structure. Upon annealing, they crystalize into anatase structure, and when temperature increases further, phase transformation occurs and anatase gradually turns into rutile. The morphology of the nanotube retains at the amorphous and anatase phase, but entirely collapses when rutile phase dominates. Figure 6 shows the morphology evolution of TiO2 NT undergoing phase transformation [37]. The tubular structure gradually collapses and fused into bulky columns. The phase transformation in nanostructured TiO2 has been reasonably well studied. Although the exact temperature for phase transformation to occur depends on material morphology, the anatase to rutile transfor-

Figure 6. Scanning electron microscopy (SEM) images of TiO2NT at different calcination temperatures. (a) as-prepared

(inset is the cross section), (b) 500C, (c) 650C, and (d) 800C. (Adapted from Ref. [37]).

3. Phase transformation and luminescence from TiO2 nanotubes

mation is the common outcome when the calcination temperature increases.

excitation energy can be tracked. This is the 2D XANES-XEOL.

198 Titanium Dioxide - Material for a Sustainable Environment

Upon X-ray excitation, TiO2 NT emits light. The luminescence properties of TiO2 NT depend on both the crystal phases of the sample as well as the excitation energy. For nanostructures, it also depends on the size since the complete thermalization path in nanostructures can be truncated if the nanostructure is smaller than the thermalization path which can be tracked by the universal kinetic energy-dependent escape depth of electrons. For site specificity, we record the optical spectra (i.e. XEOL) using a fixed excitation energy just about the edge of interest, so that the site of interest can be preferentially excited. For example, XEOL can be recorded by selecting excitation energy above the O K-edge at 580 eV. In this circumstance, both the Ti 2p electrons and the O 1 s electrons can be excited, and more core holes can be thermalized, opening up both site-specific optical channels.

Figure 8 shows the XEOL profile of TiO2 NT calcinated at elevated temperatures. Visible luminescence at the green region starts to appear when amorphous TiO2 NT begins to crystalize into anatase, and the intensity gradually increases as the crystallinity improves. A minor high energy shift is also seen as the calcination temperature increases. The emergence of rutile phase is accompanied by an intense near-IR emission [37]. Its intensity increases with increased rutile concentration but the wavelength persists. The observation is in agreement with the ones reported using conventional laboratory PL [21, 22]. Both emissions are below the

Figure 7. (a) XANES spectra of as-prepared and calcinated TiO2 NT and (b) crystal phases' composition derived from linear combination fit.

with excitation energy across the Ti L3,2- edge. This is done by the following: at each energy step, sum up the total optical photon counts (zero order PLY) or the photon counts within a selected wavelength window (partial PLY), and the final spectrum is a photon counts versus excitation energy plots. We can then compare the PLY spectra with the corresponding XANES spectra to obtain the relationship between structure and luminescence, so PLY is also called as

Luminescence from TiO2 Nanotubes and Related Nanostructures Investigated Using Synchrotron X-Ray…

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When TiO2 NT only contains one emission peak at the green region, the PLY has the same profile as XANES, meaning that the radiative recombination site originates from the hybridized Ti 3d and O 2p states, and the intensity of the luminescence is proportional to the absorption coefficient of the element probed. This means, the decay of Ti 2p hole contributes to the luminescence positively. However, the PLY of the near IR emission from rutile retains all

The inversion of PLY is not an unusual phenomenon. XEOL is produced by electron hole recombination near the bandgap via excitonic transition or energy transfer to defect states, but they are produced by thermalization following the decay of a core hole. The luminescence intensity can decrease if the energy transfer to the optical channel becomes less efficient, for example, the sampling depth decreases (increasing absorption) abruptly above the edge reducing the thermalization path. This is not uncommon for nanostructures since at these soft X-ray excitations, the specimen absorbs all the X-rays and above the absorption edge, the penetration depth of the X-ray reduces markedly, for example, just above and below the Ti L3 edge, the one absorption length reduces by a factor of 6.5 meaning that above the edge most of the X-rays are absorbed in the near surface region (0.13 μm) as compared to the penetration depth just below (0.85 μm).<sup>1</sup> As a result, at excitation energy above the edge, a significant fraction of the energetic electrons created at or near the surface escape the sample without contributing fully to thermalization. If the luminescence has a bulk origin, like a deep level defect, hitting the

Following this theory, the visible green band can be attributed to a surface-related defect of anatase origin, while the near IR emission is from bulk defect of rutile. This is also true when TiO2 NT has more than one type of defect, as can be seen from the TiO2 NT containing mixed phases, both visible green and near IR emission is present (Figure 9b). If we track each emission band individually using wavelength-selected PLY, we are able to distinguish between the optical responses from each emission band. As shown in Figure 9b, although the overall PLY exhibits a total inversion, the visible green component maintains the positive response. This means that the two emission bands have different origin and are from different region of the specimen. The surface-related defect is from the anatase phase, once it converts to rutile, only bulk defect is present. As a result, by using optical XANES (both total PLY and wavelength-selected PLY), one can investigate multicomponent emission separately and track

Since the degree of PLY inversion is thickness dependent, we can also correlate the observed emission to the location of the sample. In the abovementioned example, TiO2 NT gradually

the "optical XANES".

the features in XANES but completely inverted.

absorption edge will decrease the luminescence.

Data obtained from X-ray calculator: http://henke.lbl.gov/optical\_constants/

the luminescence to its origin.

1

Figure 8. XEOL spectrum of TiO2NT calcinated at different temperatures. (Adapted from Ref. [37]).

bandgap of anatase and rutile, so they are of defect origin. The visible emission is usually attributed to the oxygen vacancies as well as surface hydroxyl groups in the TiO2 lattice, while the near-IR emission from rutile is from an intrinsic defect that the deep electron trap recombines with holes [22, 38]. Luminescence defected using X-ray as excitation source is similar to the one reported using laboratory PL; therefore, the experimental results obtained using the two techniques are comparable. However, XEOL can characterize luminescence from a second dimension, that is, for a fixed sample, tracking the evolution of luminescence intensity as a function of excitation energy.

Figure 9 shows the luminescence response of TiO2 NT calcinated at 400, 600, and 700C leading to the formation of anatase, mixed anatase and rutile, and rutile phase, respectively

Figure 9. PLY spectra at the Ti L3,2-edge in comparison with XANES obtained in TEY mode. (a) 400C, (b) 600C, and (c) 700C. (Adapted from Ref. [37]).

with excitation energy across the Ti L3,2- edge. This is done by the following: at each energy step, sum up the total optical photon counts (zero order PLY) or the photon counts within a selected wavelength window (partial PLY), and the final spectrum is a photon counts versus excitation energy plots. We can then compare the PLY spectra with the corresponding XANES spectra to obtain the relationship between structure and luminescence, so PLY is also called as the "optical XANES".

When TiO2 NT only contains one emission peak at the green region, the PLY has the same profile as XANES, meaning that the radiative recombination site originates from the hybridized Ti 3d and O 2p states, and the intensity of the luminescence is proportional to the absorption coefficient of the element probed. This means, the decay of Ti 2p hole contributes to the luminescence positively. However, the PLY of the near IR emission from rutile retains all the features in XANES but completely inverted.

The inversion of PLY is not an unusual phenomenon. XEOL is produced by electron hole recombination near the bandgap via excitonic transition or energy transfer to defect states, but they are produced by thermalization following the decay of a core hole. The luminescence intensity can decrease if the energy transfer to the optical channel becomes less efficient, for example, the sampling depth decreases (increasing absorption) abruptly above the edge reducing the thermalization path. This is not uncommon for nanostructures since at these soft X-ray excitations, the specimen absorbs all the X-rays and above the absorption edge, the penetration depth of the X-ray reduces markedly, for example, just above and below the Ti L3 edge, the one absorption length reduces by a factor of 6.5 meaning that above the edge most of the X-rays are absorbed in the near surface region (0.13 μm) as compared to the penetration depth just below (0.85 μm).<sup>1</sup> As a result, at excitation energy above the edge, a significant fraction of the energetic electrons created at or near the surface escape the sample without contributing fully to thermalization. If the luminescence has a bulk origin, like a deep level defect, hitting the absorption edge will decrease the luminescence.

Following this theory, the visible green band can be attributed to a surface-related defect of anatase origin, while the near IR emission is from bulk defect of rutile. This is also true when TiO2 NT has more than one type of defect, as can be seen from the TiO2 NT containing mixed phases, both visible green and near IR emission is present (Figure 9b). If we track each emission band individually using wavelength-selected PLY, we are able to distinguish between the optical responses from each emission band. As shown in Figure 9b, although the overall PLY exhibits a total inversion, the visible green component maintains the positive response. This means that the two emission bands have different origin and are from different region of the specimen. The surface-related defect is from the anatase phase, once it converts to rutile, only bulk defect is present. As a result, by using optical XANES (both total PLY and wavelength-selected PLY), one can investigate multicomponent emission separately and track the luminescence to its origin.

Since the degree of PLY inversion is thickness dependent, we can also correlate the observed emission to the location of the sample. In the abovementioned example, TiO2 NT gradually

bandgap of anatase and rutile, so they are of defect origin. The visible emission is usually attributed to the oxygen vacancies as well as surface hydroxyl groups in the TiO2 lattice, while the near-IR emission from rutile is from an intrinsic defect that the deep electron trap recombines with holes [22, 38]. Luminescence defected using X-ray as excitation source is similar to the one reported using laboratory PL; therefore, the experimental results obtained using the two techniques are comparable. However, XEOL can characterize luminescence from a second dimension, that is, for a fixed sample, tracking the evolution of luminescence inten-

Figure 8. XEOL spectrum of TiO2NT calcinated at different temperatures. (Adapted from Ref. [37]).

Figure 9 shows the luminescence response of TiO2 NT calcinated at 400, 600, and 700C leading to the formation of anatase, mixed anatase and rutile, and rutile phase, respectively

Figure 9. PLY spectra at the Ti L3,2-edge in comparison with XANES obtained in TEY mode. (a) 400C, (b) 600C, and (c)

sity as a function of excitation energy.

200 Titanium Dioxide - Material for a Sustainable Environment

700C. (Adapted from Ref. [37]).

<sup>1</sup> Data obtained from X-ray calculator: http://henke.lbl.gov/optical\_constants/

transform from anatase to rutile at elevated temperature, with the disappearing of the visible luminescence and emerging of the near IR emission. The PLY of visible emission (anatase phase) always exhibits a positive jump, meaning that anatase phase remains on the surface. On the other hand, as soon as rutile appears, it has an inverted PLY. If rutile appears on the surface of the tube, the thickness effect should be avoided, and PLY should have a normal positive jump. The observation suggests that the phase transformation of TiO2 NT during calcination starts from the bottom to the top, from inside to the outside. This conclusion is later confirmed using scanning transmission X-ray microscopy that measures XANES from the top and the bottom of nanotubes separately in a mixed phase TiO2 NT [39].
