XANES Results

During XANES measurement, the incident X-ray beam was irradiated directly to the surface of samples. Moreover, X-ray beam was only absorbed by the surface layer sample.

Therefore, the XANES results describe the characteristics of the surface layer of sample.

When the TNAs were irradiated by laser in parallel mode, the surface layer of sample was annealed by laser first and following by internal parts of sample. Hence, the XANES results will only explain the properties of the surface layer of sample annealed by laser.

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Ti L2,3 edge XANES spectra of TiO2 nanotube arrays prepared by anodic oxidation in NH₄F solution, subsequently annealed by excimer laser at different fluences of 0.042 Jcm^-2 and 0.1 Jcm^-2 at 9000 shots are shown in Figure 4-9. As seen in Figure 4-9, there are four dominant features, which can be attributed to excitations of Ti 2p_3/2 (L₃ edge) and Ti 2p_1/2 (L₂ edge) core levels into empty Ti 3d states. For TiO2 nano-tube arrays as-grown in NH4F solution at room temperature, the L_2,3-edge shows broad features with low intensities in t_2g and structureless eg, which are indicative of amorphous TiO2 [83]. When as- grown TiO2 was annealed by laser, the spectra instead show definite crystalline structures as indicated by the sharpness and higher t2g orbitals and double features of eg orbitals in L3 edge. Moreover, the leading edge of TNAs annealed by laser also has shifted to higher energy (0.6 eV). It indicates that Ti has changes from lower charge state Ti⁰, Ti⁺², Ti⁺³ to Ti⁴⁺ (TiO2) [84, 85]. This suggested that TNAs prepared at room temperature contain not only amorphous TiO₂ but also some impurities such as TiO, Ti2O3 and after annealing by laser, the impurities TiO, Ti2O3

were transferred to TiO₂, thus causing the Ti L-edge shift to higher energy.

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Table 4-2 summarizes the intensity ratios of orbitals for Ti L3 edge of TNAs prepares by anodic oxidation as grown in NH4F solution, subsequently annealed by laser at different fluences of 0.042 Jcm^-2 and 0.1 Jcm^-2 at 9000 shots

40 -t2g)/ I(L3-eg) increases from 0.9 for as-grown sample, to 1.02 and 1.04 as samples annealed by laser at 0.042 and 0.1 J.cm^-2 of fluence, respectively. Because the intensity of the L-edge features varies with the density of empty d-states, an increase of I(L3-t2g)/ I(L3-eg) intensity ratio implies an empty state in t2g orbitals, which is consistent with an increase in Ti⁴⁺ cations.

The eg related peak of the L3 edge is split into two peaks at 461 eV as the fluence of 0.042 J.cm^-2 and above. This shows that the major difference between Ti L2,3 spectra of amorphous and crystalline phases of TiO2 is significant change in positions, intensities and widths of eg -related peak b1 and b2. For example, with anatase peak b1 is substantially stronger than that of peak b2; while with rutile, the intensity of peak b2 is substantially stronger than that of peak b1. In addition, when the fluences is increased from 0.042 to 0.1 J.cm^-2, the I(b1)/ I(b2) intensity ratio is increased 1.04 and 1.05. It means that TNAs annealed by laser with increasing fluence, TNAs are transferred to TiO2 anatase [86].

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Oxygen 1s (K-edge) absorption edges, which consist of the excitation of an oxygen 1s electron into unoccupied 2p-states, are employed to map out the oxygen 2p-projectd density of unoccupied states in order to examine the oxidation states and crystalline phases of TNAs.

The O K-edgeXANES spectra of TiO₂ nanotube arrays prepared by anodic oxidation in NH₄F solution: As-grown, and annealed by excimer laser at different fluences of 0.042 Jcm^-2 and 0.1 Jcm^-2 at 9000 shots as shown in Figure 4-10.

In general, there are two dominant features in lower energy, 530-536 eV in the O K-edge spectra, which can be assigned to the transitions into Ti 3d t_2g and e_g levels. Broad peaks are observed for as-grown TNAs due to slight variation in bond lengths and angles, in hearent to spectra of amorphous TiO₂ solid [82]. However, after annealing by laser at different fluences of 0.042 Jcm^-2 and 0.1 Jcm^-2, the Ti 3d t2g and eg features become more distinct and narrow shapes of Ti and new peak at higher energy (539.3 eV), which can be attributed to transitions into 4sp orbitals, indicative of anatase phase TiO2 [85]. In addition, it is interesting to note that the energy separation of both t_2g and e_g orbitals provides a direct measure of ligand-field splitting (LFS) of TiO2. The LFS can be obtained from the corresponding oxygen k-edge spectra, revealing a decrease in sequence of TiO₂ (2.6-2.7 eV), Ti₂O₃ (2.2 eV), and TiO (1.5 eV) [87, 88]. The spectrum of as-grown sample reveals a ligand-field splitting of 2.2 eV.

Upon annealing by laser at fluences of 0.042 Jcm^-2 and 0.1 Jcm^-2, LFS increases to 2.6 eV. It indicates that the lower LFS for as-grown sample is due to remaining charge transfer to Ti⁴⁺. On the other words, as-grown sample still contain some impurities and annealing by laser there is a charge transfer from Ti⁺², Ti⁺³ to Ti⁴⁺ (TiO2).

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528 531 534 537 540 543 546 549 552 2.2 oxidation in NH4F solution: as-grown and annealed by by laser at different fluences of 0.042 Jcm^-2 and 0.1 Jcm^-2 at 9000 shots

Figure 4-11 shows the fitting results for O K-edge XANES spectrum of as-grown TNAs by anodic oxidation in NH4F solution. As seen in the Figure 4-11, the spectra shows that as-grown TNAs at room temperature processes 82 % TiO2 amorphous phase, 11 % Ti2O3, and 7 % TiO. The low percentage of amorphous TNAs prepared in NH₄F solution can be attributed to low oxygen ion formation from only 3 wt% H2O addition. After that, the TNAs annealed by laser at fluences of 0.042 J.cm^-2 and 0.1 J.cm^-2 or post- annealed at 400^oC.

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Figures 4-12 and 4-13 show the fitting results for O K-edge XANES spectra of TNAs annealed by laser at fluences of 0.042 J.cm^-2 and 0.1 J.cm^-2, respectively. As seen in Figure 4-12, the phase transformation has taken place from amorphous to anatase phase (81 %), and particularly all of cations 11 % Ti³⁺ (Ti₂O₃), and 7 % Ti²⁺ (TiO) are transferred completely to Ti⁴⁺ (TiO2) after laser treatment at 0.042 J.cm^-2 of fluence. Further increase of fluence to 0.1 J.cm^-2, the percentage of crystallinity for anatase rose to 90 % (seeFigures 4-13). Therefore, the increase in fluence from 0.042 J.cm^-2 to 0.1 J.cm^-2, the percentage of crystallinity of TNAs has increased from 80 % to 90 % for anatase phase. On the other hand, the fitting results for O K-edge XANES spectrum of TNAs post-annealed at 400^oC (Figures 4-14) shows that 4%

Ti₂O₃ impurity still exists in sample after post annealing. Furthermore, the percentage of TiO₂ anatase phase was limited to 86%.

Table 4-3 summarizes the amount of crystalline, amorphous phase and impurities of TiO₂ nanotube arrays as-grown, after laser annealing with 0.042 J.cm^-2 and 0.1 J.cm^-2 at 9000 shots, and post annealing at 400^oC. First, in the as-grown TiO₂ nanotube arrays prepared in NH₄F solution with low H2O content, at room temperature there is 18 % impurities (11 % Ti³⁺

(Ti₂O₃), and 7 % Ti²⁺ (TiO) cations) and 82 % amorphous. As the annealing temperature is increased to 400^oC, this additional energy enable phase transformation from amorphous phase to anatase phase (86%), and also reduced the impurities from 18% to 4% by charge transferred for Ti cations. However, in the part of TNAs irradiated by laser at 0.042 J.cm^-2 of fluence, there is not only the phase transformation happened from amorphous to anatase phase, but also all of impurities TiO and Ti2O3 were transferred to TiO2. In addition, further increase of fluence to 0.1 J.cm^-2, the crystallinity of TiO₂ nanotube has increased to 90% for anatase phase. The above results prove that ELA is powerful technique to make charge transferred for Ti cations from lower charge state Ti⁺² or Ti⁺³ to Ti⁴⁺ as well as improve the crystalinity of amorphous TiO2.

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Figure²¹4-11 Fitting results for O K-edge XANES spectrum of as-grown TNAs

528 531 534 537 540 543 546 549 552

O K edge

Intensity (a.u. )

Photon Energy (ev)

Laser annealing F = 0.042 Jcm^-2 Anatase TiO₂ 81 % Amorphous TiO₂ 19 % r²=0.9872

Figure²²4-12 Fitting results for O K-edge XANES spectrum of TNAs annealed by laser with fluence of 0.042 Jcm^-2 at 9000 shots

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528 531 534 537 540 543 546 549 552

O K edge

Intensity (a.u. )

Photon Energy (ev)

Laser annealing F = 0.1 Jcm^-2 Anatase TiO₂ 90 % Amorphous TiO₂ 10 % r²=0.9772

Figure²³4-13 Fitting results for O K-edge XANES spectrum of TNAs annealed by laser with fluence of 0.1 Jcm^-2 at 9000 shots

Figure²⁴4-14 Fitting results for O K-edge XANES spectrum of TNAs annealed at 400^o C

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Percentage of Polymorphs TiO₂ and impurities (%)

TiO Ti2O3 Amorphous TiO2 Anatase TiO2

4.5 The Improvement of Crystallinity and the Reduction of Surface Damage of TNAs

在文檔中 準分子雷射處理對於二氧化鈦奈米管陣列之型態與結構之影響 (頁 52-61)