2.1. Electrochemical anodization

In TiO2, Ti and O form distorted octahedra, and the different octahedra packing leads to various crystal phases, such as anatase, rutile, brookite, and B-phase. Anatase is a low-density metastable phase, and contains catalytically active crystal facets, and a relatively loose structure. Rutile is the most thermodynamically stable phase with the highest density. Brookite is less common, although it has been reported that it is an intermediate phase during phase transformation [6]. B-phase has a highly distorted structure, which commonly occurs during hydrothermal synthesis, and is a good candidate as ion battery electrode [7, 8]. The crystal phases are highly sensitive to temperature, and with increasing temperature, phase transformation occurs. Under mild calcination temperature (200–600C), TiO2 NT forms an anatase phase, while higher calcination temperature induces an irreversible phase transformation to rutile [9, 10]. Studies on various TiO2 nanostructures, such as nanoparticles, nanowires, thin films, suggest that the phase transformation temperature highly depends on the morphology and composition of materials [11–13]. In fact, with well-controlled synthesis, an anatase-rutile composite can provide better catalytic performance than single-phase TiO2 [14, 15]. TiO2 nanotubes produced electrochemically from a Ti foil are usually in an amorphous form and transform to anatase phase with calcination temperature below 600C. Although the exact transformation temperature can be adjusted by nanotube size or introducing dopant, at higher temperature, all anatase TiO2 eventually trans-

Synthesis strategies of TiO2 nanotubes have been developed for more than a decade, and they can now be synthesized with well-controlled diameters, lengths, wall thicknesses, and surface roughness [16, 17]. Hydrothermal, anodization, and template-assisted methods are successful in producing TiO2 NT. Besides, some complex structures such as tube-in-tube [18], particledecorated [19], and hierarchical structures [20] have also been reported. Such structures exhibit unusual electronic and optical properties and their potential applications in various fields are

The luminescence properties of TiO2 are strongly dependent on crystal phases and the quality of the crystallinity. For example, upon X-ray excitation, anatase usually exhibits a visible green emission at around 550 nm, while rutile shows an intense near-IR emission at around 800 nm. Both emissions have energies lower than the bandgap, so they both are of a defect origin. The visible green emission is highly sensitive to surface structure and the presence of impurities, and is affected by modifying the surface condition of TiO2 [21, 22], while the near IR emission is relatively stable. In addition, a recent work reports that weak bandgap emission has been observed from ultra-small amorphous TiO2 nanograss structures (unzipped TiO2 NT) [20]. It is thus crucial to establish a connection between the observed luminescence and the structure of

The conventional way of detecting luminescence is photoluminescence, PL (often called fluorescence), which uses visible or UV light as an excitation source. In wide bandgap semiconductors such as TiO2, valence electrons are excited to the conduction band, and radiative recombination is recorded with a spectrometer. The experiment setup is readily available, and it is a direct probe of the emission from a sample. However, since the electrons are from the entire valence band, it lacks element specificity. In other words, we know at what wavelength the sample is emitting light, but we do not know what is responsible for it. In this chapter, we

forms to the thermodynamically stable rutile phase.

192 Titanium Dioxide - Material for a Sustainable Environment

still actively explored.

the materials.

TiO2 NT discussed in this chapter is synthesized using electrochemical anodization, which is a well-developed synthetic strategy for producing highly oriented TiO2 nanotube arrays grown on Ti foils. Figure 1a shows a schematic layout of the anodization setup. A piece of Ti foil is used as the anode, and a piece of Pt foil is used as the cathode. The two electrodes are immersed in an ethylene glycol-based electrolyte which contains F (from either HF or NH4F), and a voltage of several tens of volts is applied [17]. After certain duration, TiO2 NT will form on the Ti foil, with a typical morphology, shown in Figure 1b. By controlling the electrolyte composition, voltage, and the duration of the anodization, TiO2 NT can be produced with controlled diameter, length, and wall thickness. The as-formed TiO2 NT has an amorphous structure, so post-annealing is often required to produce TiO2 NT of desired crystal phases.

Figure 1. (a) Experimental setup of anodization and (b) a typical top-view SEM image of as-prepared TiO2NT.

#### 2.2. X-ray absorption near-edge structure

X-ray absorption near-edge structure (XANES) is a technique that probes the local chemical environment of an element in a compound. It utilizes a tunable X-ray source from synchrotron to excite a core electron of the element of interest and monitor the modulation of the absorption coefficient at the absorption edge. Once the energy of the incoming X-ray is sufficient to induce a core electron transition, there is a sharp increase in the absorption coefficient, which is called an "edge" in the corresponding absorption spectrum. The energy position of the "edge" marks the onset energy where electronic transition occurs. Above the edge, spectrum usually exhibits rich features, which are due to multiple scattering between the outgoing electron and the electrons from surrounding atoms. These features are regarded as the "fingerprint" of the material, since they are highly sensitive to the local geometry and the species of neighboring atoms. In short, the photoelectron propagates away from the absorbing atom, samples the neighboring atoms via backscattering, brings the information back to the absorbing atom, the information appears as spectral features in the XANES. In recent years, XANES has been recognized as a valuable technique and has been used to analyze various novel nanomaterials used in catalysis [25], solar cells [26], batteries [27], and so on. It can provide element specific chemical structure with both surface and bulk sensitivity by selecting different detection modes (i.e. total electron yield (TEY) for measuring a few angstroms to nanometers on the surface, and fluorescence yield (FY) for measuring several hundreds of nanometers (as for TiO2)) with soft X-rays and a few micrometers under the surface. This advantage makes XANES an ideal non-destructive probe for analyzing layered, core-shell, or heterostructures. In addition, it can be combined with in situ facility to study the involvement of chemical species during chemical reaction in catalysis [28] or electrochemical cycling [29]. When using a micro- or nanosized probing beam, XANES can also be conducted with site sensitivity, to achieve chemical structure mapping of soil [30], biofilms [31], or nanocomposite [32].

using a molecular orbital perspective. That is, in the octahedral field, the 3d orbitals are divided into two energy levels, t2g and eg, and they correspond to the a (a') and b (b') peaks, respectively. The eg peak exhibits further splitting due to the reduced symmetry of Oh in anatase and rutile. For anatase, it is reduced to a D2d symmetry, and for rutile, a D2h symmetry, and in XANES, the difference is identified by the intensity ratio of the eg doublet: anatase has a

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The difference in crystal structure is also seen at the Ti L2-edge. Due to the lifetime broadening and the background from the L3 edge, the Ti L2-edge does not exhibit features that are as sharp as the ones at the L3-edge. However, we can still see that the b' peak maximum shifts, so that

Based on these features, one could perform a composition analysis on any TiO2 material, which contains a mixed phase of anatase and rutile using linear combination fit. We illustrate

XEOL is an X-ray photon-in, optical photon-out technique. It monitors the luminescence from a material under X-ray excitation. It differs from laboratory photoluminescence by its excitation source. When X-ray is used, electrons at deeper level are excited, leaving core hole, instead of just valence hole behind as in PL. The core hole is then filled with electrons at shallower levels, producing Auger electrons and a hole at an outer shell. The energetic electrons continue to travel in the solid, losing its energy and producing shallower core holes. This cascade process, often called "thermalization", is repeated until the hole is created at the top of valence

stronger peak b1 and rutile has a stronger peak b2.

this with examples in the next section.

2.3. X-ray excited optical luminescence (XEOL)

the distance between a' and b' is larger in rutile than in anatase.

Figure 2. Typical XANES spectra of anatase and rutile TiO2 at the Ti L3,2-edge.

The rich information in XANES is obtained by excitation of core electrons in the element of interest and at appropriate energy levels. As for TiO2, XANES collected at three edges are of particular interest. The Ti L3,2-edge is the excitation of Ti 2p electrons (2p3/2 for L3, and 2p1/2 for L2) to the previously unoccupied electronic states with d and s characters. The Ti K-edge, which is the excitation of Ti 1s electron, and the O K-edge, which excites the O 1s electron into previously unoccupied p states. Since the conduction band of TiO2 is mainly composed of hybridized Ti 3d and O 2p states, which makes the Ti L3,2- and the O K-edges ideal for the investigate the structure and bonding, hence chemical reactivity of TiO2. The Ti K-edge, on the other hand, is also used in identifying Ti-based species, but the spectra analysis strongly focuses on the pre-edge features, which is the 1s to 3d dipole forbidden, quadruple transition [33, 34]. Besides, the Ti K-edge is more often used to study the bonding information by looking at absorption features well-above the edge (i.e. the extended X-ray absorption fine structure) [27, 34]. Herein, we limit our discussion on the Ti L3,2-edge.

Figure 2 shows typical XANES spectra of TiO2 at the Ti L3,2-edge of anatase and rutile. In a Ti L3,2-edge XANES, there are two sets of peaks, which belong to the L3- and L2-edge features. In each set, there is a sharp peak at lower energy (a and a') and a broad peak at higher energy (b and b'). Due to crystal field splitting, the conduction band of TiO2 can actually be interpreted

Luminescence from TiO2 Nanotubes and Related Nanostructures Investigated Using Synchrotron X-Ray… http://dx.doi.org/10.5772/intechopen.72856 195

Figure 2. Typical XANES spectra of anatase and rutile TiO2 at the Ti L3,2-edge.

2.2. X-ray absorption near-edge structure

194 Titanium Dioxide - Material for a Sustainable Environment

X-ray absorption near-edge structure (XANES) is a technique that probes the local chemical environment of an element in a compound. It utilizes a tunable X-ray source from synchrotron to excite a core electron of the element of interest and monitor the modulation of the absorption coefficient at the absorption edge. Once the energy of the incoming X-ray is sufficient to induce a core electron transition, there is a sharp increase in the absorption coefficient, which is called an "edge" in the corresponding absorption spectrum. The energy position of the "edge" marks the onset energy where electronic transition occurs. Above the edge, spectrum usually exhibits rich features, which are due to multiple scattering between the outgoing electron and the electrons from surrounding atoms. These features are regarded as the "fingerprint" of the material, since they are highly sensitive to the local geometry and the species of neighboring atoms. In short, the photoelectron propagates away from the absorbing atom, samples the neighboring atoms via backscattering, brings the information back to the absorbing atom, the information appears as spectral features in the XANES. In recent years, XANES has been recognized as a valuable technique and has been used to analyze various novel nanomaterials used in catalysis [25], solar cells [26], batteries [27], and so on. It can provide element specific chemical structure with both surface and bulk sensitivity by selecting different detection modes (i.e. total electron yield (TEY) for measuring a few angstroms to nanometers on the surface, and fluorescence yield (FY) for measuring several hundreds of nanometers (as for TiO2)) with soft X-rays and a few micrometers under the surface. This advantage makes XANES an ideal non-destructive probe for analyzing layered, core-shell, or heterostructures. In addition, it can be combined with in situ facility to study the involvement of chemical species during chemical reaction in catalysis [28] or electrochemical cycling [29]. When using a micro- or nanosized probing beam, XANES can also be conducted with site sensitivity, to

achieve chemical structure mapping of soil [30], biofilms [31], or nanocomposite [32].

[27, 34]. Herein, we limit our discussion on the Ti L3,2-edge.

The rich information in XANES is obtained by excitation of core electrons in the element of interest and at appropriate energy levels. As for TiO2, XANES collected at three edges are of particular interest. The Ti L3,2-edge is the excitation of Ti 2p electrons (2p3/2 for L3, and 2p1/2 for L2) to the previously unoccupied electronic states with d and s characters. The Ti K-edge, which is the excitation of Ti 1s electron, and the O K-edge, which excites the O 1s electron into previously unoccupied p states. Since the conduction band of TiO2 is mainly composed of hybridized Ti 3d and O 2p states, which makes the Ti L3,2- and the O K-edges ideal for the investigate the structure and bonding, hence chemical reactivity of TiO2. The Ti K-edge, on the other hand, is also used in identifying Ti-based species, but the spectra analysis strongly focuses on the pre-edge features, which is the 1s to 3d dipole forbidden, quadruple transition [33, 34]. Besides, the Ti K-edge is more often used to study the bonding information by looking at absorption features well-above the edge (i.e. the extended X-ray absorption fine structure)

Figure 2 shows typical XANES spectra of TiO2 at the Ti L3,2-edge of anatase and rutile. In a Ti L3,2-edge XANES, there are two sets of peaks, which belong to the L3- and L2-edge features. In each set, there is a sharp peak at lower energy (a and a') and a broad peak at higher energy (b and b'). Due to crystal field splitting, the conduction band of TiO2 can actually be interpreted using a molecular orbital perspective. That is, in the octahedral field, the 3d orbitals are divided into two energy levels, t2g and eg, and they correspond to the a (a') and b (b') peaks, respectively. The eg peak exhibits further splitting due to the reduced symmetry of Oh in anatase and rutile. For anatase, it is reduced to a D2d symmetry, and for rutile, a D2h symmetry, and in XANES, the difference is identified by the intensity ratio of the eg doublet: anatase has a stronger peak b1 and rutile has a stronger peak b2.

The difference in crystal structure is also seen at the Ti L2-edge. Due to the lifetime broadening and the background from the L3 edge, the Ti L2-edge does not exhibit features that are as sharp as the ones at the L3-edge. However, we can still see that the b' peak maximum shifts, so that the distance between a' and b' is larger in rutile than in anatase.

Based on these features, one could perform a composition analysis on any TiO2 material, which contains a mixed phase of anatase and rutile using linear combination fit. We illustrate this with examples in the next section.
