2.3. X-ray excited optical luminescence (XEOL)

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 band and fully thermalized electrons settling in the bottom of the conduction band. Then, the valence holes recombine with electrons at the bottom of the conduction band, producing luminescence via radiative excitonic de-excitation (near bandgap emission) and energy transfer to the impurity and defect states in the band gap (defect emission). Figure 3 illustrates the process.

Since the energy of core electrons is element specific, XANES allows to track the absorption behavior of an element to provide element-specific local environment, a combined XEOL-XANES technique will allow us to build the relationship between the observed luminescence and the type of element that is responsible for it, and predict the chemical environment that leads to such luminescence. This is particularly valuable when analysing the role of dopant or structural defect luminescence [35, 36], as well as identifying the light-emitting component in a mixture of materials [24, 32].

Figure 4. Experimental setup for XANES-XEOL measurements at synchrotron beamline endstation.

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Figure 5. 2D XANES-XEOL map of TiO2 nanowires. The x-axis is the luminescence wavelength, and the y-axis is the excitation photon energy. Color-coded z-axis is the luminescence intensity. A horizontal cut from the map (top panel) is XEOL at selected excitation energy and a vertical cut (right panel) is optical XANES of a selected luminescence

wavelength.

Figure 4 shows a schematic layout of a synchrotron radiation endstation for conducting such experiment. We scan the incoming X-ray energy from below to above the absorption edge, the electron current generated by photoelectrons and secondary electrons is recorded as TEY, the emitted X-ray fluorescence photon is recorded as FY, and the amount of optical photon generated due to valence hole-conduction electron radiative recombination is recorded as photoluminescence yield (PLY). PLY can be collected for the entire wavelength region (200– 900 nm) or at selected wavelengths. Using an energy/wavelength dispersive spectrometer, we could further gain a luminescence spectrum at each energy step. This will generate a 3D graph, which contains both the absorption behavior together with the associated luminescence. As shown in Figure 5, the color-coded map has a wavelength and excitation energy axis, the

Figure 3. Illustration of a XEOL process from shallow core levels.

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Figure 4. Experimental setup for XANES-XEOL measurements at synchrotron beamline endstation.

band and fully thermalized electrons settling in the bottom of the conduction band. Then, the valence holes recombine with electrons at the bottom of the conduction band, producing luminescence via radiative excitonic de-excitation (near bandgap emission) and energy transfer to the impurity and defect states in the band gap (defect emission). Figure 3 illustrates the

Since the energy of core electrons is element specific, XANES allows to track the absorption behavior of an element to provide element-specific local environment, a combined XEOL-XANES technique will allow us to build the relationship between the observed luminescence and the type of element that is responsible for it, and predict the chemical environment that leads to such luminescence. This is particularly valuable when analysing the role of dopant or structural defect luminescence [35, 36], as well as identifying the light-emitting component in a

Figure 4 shows a schematic layout of a synchrotron radiation endstation for conducting such experiment. We scan the incoming X-ray energy from below to above the absorption edge, the electron current generated by photoelectrons and secondary electrons is recorded as TEY, the emitted X-ray fluorescence photon is recorded as FY, and the amount of optical photon generated due to valence hole-conduction electron radiative recombination is recorded as photoluminescence yield (PLY). PLY can be collected for the entire wavelength region (200– 900 nm) or at selected wavelengths. Using an energy/wavelength dispersive spectrometer, we could further gain a luminescence spectrum at each energy step. This will generate a 3D graph, which contains both the absorption behavior together with the associated luminescence. As shown in Figure 5, the color-coded map has a wavelength and excitation energy axis, the

process.

mixture of materials [24, 32].

196 Titanium Dioxide - Material for a Sustainable Environment

Figure 3. Illustration of a XEOL process from shallow core levels.

Figure 5. 2D XANES-XEOL map of TiO2 nanowires. The x-axis is the luminescence wavelength, and the y-axis is the excitation photon energy. Color-coded z-axis is the luminescence intensity. A horizontal cut from the map (top panel) is XEOL at selected excitation energy and a vertical cut (right panel) is optical XANES of a selected luminescence wavelength.

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

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.

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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

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

thermalized, opening up both site-specific optical channels.

linear combination fit.
