1. Introduction

The properties and potential applications of TiO2 nanotubes (TiO2NT) have been actively researched. Because of their 1D tubular structure, they provide an ideal media for highly directional charge transport, and are explored in many photoelectrochemical applications such as solar cells [1], photocatalysis [2], sensors [3], and battery electrodes [4]. Besides, the high biocompatibility of TiO2 also makes it an excellent coating for substrate for depositing bioactive species [5].

> © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

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 transforms to the thermodynamically stable rutile phase.

highlight a characterization technique that probes luminescence of materials with element and site specificity: X-ray absorption near-edge structure (XANES) in combination with X-ray excited optical luminescence (XEOL). It is conducted at synchrotron radiation facility, and utilizes a tunable X-ray to monitor the luminescence from material upon core electron excitation. XANES-XEOL analysis, especially in the soft X-ray range, has proven an effective technique in studying the luminescence mechanism of various light-emitting materials, such as

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

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The content of the chapter is organized as follows: we first give an overview of the methods for producing highly oriented nanotubes by anodization, followed by an introduction of the XANES and XEOL techniques. Then, we use two sets of examples to demonstrate how XANES-XEOL method is applied in analyzing the luminescence mechanism of TiO2 NT: first is the phase transformation associated luminescence and second is the TiO2 hierarchical structures and doped structures. Finally, an outlook of this material and the use of synchrotron

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

ZnO nanowires [23], and GaN-ZnO solid solution [24].

2. Material synthesis and characterization techniques

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.

X-ray spectroscopy are given.

2.1. Electrochemical anodization

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 still actively explored.

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 materials.

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 highlight a characterization technique that probes luminescence of materials with element and site specificity: X-ray absorption near-edge structure (XANES) in combination with X-ray excited optical luminescence (XEOL). It is conducted at synchrotron radiation facility, and utilizes a tunable X-ray to monitor the luminescence from material upon core electron excitation. XANES-XEOL analysis, especially in the soft X-ray range, has proven an effective technique in studying the luminescence mechanism of various light-emitting materials, such as ZnO nanowires [23], and GaN-ZnO solid solution [24].

The content of the chapter is organized as follows: we first give an overview of the methods for producing highly oriented nanotubes by anodization, followed by an introduction of the XANES and XEOL techniques. Then, we use two sets of examples to demonstrate how XANES-XEOL method is applied in analyzing the luminescence mechanism of TiO2 NT: first is the phase transformation associated luminescence and second is the TiO2 hierarchical structures and doped structures. Finally, an outlook of this material and the use of synchrotron X-ray spectroscopy are given.
