4. Luminescence from TiO2 hierarchical nanostructures

When anodization is conducted at a high voltage (e.g. above 70 V), the nanotubular structure starts to break down due to the constant thinning of the tube walls, leading to the formation of a nanograss (NG)-nanotube (NT) hierarchical structure [20], shown in Figure 10. Although the formation of nanograss has long been observed, they are unzipped nanotube with reduced strain, and are often overlooked and treated as the by-product during TiO2 NT formation. Recent findings suggest that the NG-on-NT hierarchical membrane exhibits interesting optical properties, which bring more potential applications of this material [40].

The most interesting observation from this structure is that the band emission is observed from the amorphous NG, while NT does not exhibit bandgap luminescence. TiO2 is an indirect bandgap material, and the most commonly observed emission is from defects. A recent XEOL study on the amorphous TiO2 NG, however, detected the weak but clear luminescence from it [20]. TiO2 NG is excited with energy from below to above the Ti L3,2-edge. As shown in Figure 11, the emission contains a broad peak centered at 3.30 eV when the excitation energy is below and well above the edge, while a second peak at 3.76 eV appears only when the

energy just passes the edge. The 3.30 eV emission matches the value of bandgap measured by UV-vis absorption, and similar emission, although rare, has been reported from nanoparticles [41]. The 3.76 eV, however, is a unique band, and only appears when transition of the Ti 2p electron takes place. This band is hence a site-specific luminescence form NG and has been

Figure 11. Top panel: Ti L3,2-edge XANES of as-prepared nanograss. Bottom panel: XEOL spectra of nanograss under

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Meanwhile, with two different nanostructures present, the hierarchical nanotubes exhibit unique phase transformation-induced light emission. A 2D XANES-XEOL study tracks the luminescence of NG and NT, after calcination at different temperatures, as a function of excitation energy is shown in Figure 12 [40]. This map provides luminescence property in both energy space and wavelength space. Although anatase-to-rutile is a common trend at increased calcination temperature, NG can tolerate a much higher temperature (as high as 850C) and remain anatase-like. The representative near-IR emission for rutile only shows up when the sample is calcinated at 900C. In normal NT, a complete phase transformation

Thus, 2D XANES-XEOL provides a clear view of the evolution of the luminescence, hence the structure. A hierarchical structure, which is made of the same material but of different morphologies, exhibits independent luminescence from the top and the bottom of the membrane.

attributed to an up-conversion emission due to size effect [20].

selected excitation energies indicated in the top panel. (Adapted from Ref. [20]).

usually occurs at temperature around 650C [37].

Figure 10. SEM images of nanograss-nanotube hierarchical structure. (a) Top/side view and (b) bottom view. (Adapted from Ref. [20]).

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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

When anodization is conducted at a high voltage (e.g. above 70 V), the nanotubular structure starts to break down due to the constant thinning of the tube walls, leading to the formation of a nanograss (NG)-nanotube (NT) hierarchical structure [20], shown in Figure 10. Although the formation of nanograss has long been observed, they are unzipped nanotube with reduced strain, and are often overlooked and treated as the by-product during TiO2 NT formation. Recent findings suggest that the NG-on-NT hierarchical membrane exhibits interesting optical

The most interesting observation from this structure is that the band emission is observed from the amorphous NG, while NT does not exhibit bandgap luminescence. TiO2 is an indirect bandgap material, and the most commonly observed emission is from defects. A recent XEOL study on the amorphous TiO2 NG, however, detected the weak but clear luminescence from it [20]. TiO2 NG is excited with energy from below to above the Ti L3,2-edge. As shown in Figure 11, the emission contains a broad peak centered at 3.30 eV when the excitation energy is below and well above the edge, while a second peak at 3.76 eV appears only when the

Figure 10. SEM images of nanograss-nanotube hierarchical structure. (a) Top/side view and (b) bottom view. (Adapted

from Ref. [20]).

top and the bottom of nanotubes separately in a mixed phase TiO2 NT [39].

4. Luminescence from TiO2 hierarchical nanostructures

202 Titanium Dioxide - Material for a Sustainable Environment

properties, which bring more potential applications of this material [40].

Figure 11. Top panel: Ti L3,2-edge XANES of as-prepared nanograss. Bottom panel: XEOL spectra of nanograss under selected excitation energies indicated in the top panel. (Adapted from Ref. [20]).

energy just passes the edge. The 3.30 eV emission matches the value of bandgap measured by UV-vis absorption, and similar emission, although rare, has been reported from nanoparticles [41]. The 3.76 eV, however, is a unique band, and only appears when transition of the Ti 2p electron takes place. This band is hence a site-specific luminescence form NG and has been attributed to an up-conversion emission due to size effect [20].

Meanwhile, with two different nanostructures present, the hierarchical nanotubes exhibit unique phase transformation-induced light emission. A 2D XANES-XEOL study tracks the luminescence of NG and NT, after calcination at different temperatures, as a function of excitation energy is shown in Figure 12 [40]. This map provides luminescence property in both energy space and wavelength space. Although anatase-to-rutile is a common trend at increased calcination temperature, NG can tolerate a much higher temperature (as high as 850C) and remain anatase-like. The representative near-IR emission for rutile only shows up when the sample is calcinated at 900C. In normal NT, a complete phase transformation usually occurs at temperature around 650C [37].

Thus, 2D XANES-XEOL provides a clear view of the evolution of the luminescence, hence the structure. A hierarchical structure, which is made of the same material but of different morphologies, exhibits independent luminescence from the top and the bottom of the membrane.

of the emission wavelength are observed in Pd-loaded TiO2 NT. The presence of Pd has two roles such as modifies the defect states of TiO2 on the surface and quenches the radiative recombination channel in TiO2 by acting as an electron sink. Because of this, the photocatalysis

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TiO2 NT is a versatile material. It could be directly used in batteries, photocatalysis, solar cells on their own, and it can also serve as substrate for further decorating of nanoparticles or bioactive compounds. Understanding the electronic structure of TiO2 NT-based materials and controlling their electronic and optical properties is hence crucial for integrating them into practical application. Both anatase and rutile nanostructures have their characteristic luminescence at visible green and near IR regions, respectively. The visible green luminescence in NT is of a surface origin, and it is highly sensitive to surface defect and surface modification. Near IR emission, on the other hand, is of bulk origin of rutile. It is independent of material morphology and always appears as long as rutile phase is present, even in a mixed structure. Thus XEOL in combination with XANES is a unique and powerful technique which tracks the relationship of the observed luminescence and the specific local chemical structure that is responsible for it. It is hence useful in analyzing the luminescence origin of novel TiO2

efficiency can be improved with introducing PdNP.

5. Concluding remarks

nanostructures and related materials.

Lijia Liu<sup>1</sup> and Tsun-Kong Sham<sup>2</sup>

cell. Nature. 1991;353:737

Nature. 1972;238:37

rials. 2003;15:624

\*Address all correspondence to: tsham@uwo.ca

\*

1 Jiangsu Key Laboratory for Carbon-Based Functional Materials and Devices, Institute of

[1] O'Regan B, Gratzel M. Synthesis of nanocrystalline titanium dioxide dye sensitized solar

[2] Fujishima A, Honda K. Electrochemical photolysis of water at a semiconductor electrode.

[3] Varghese OK, Gong D, Paulose M, Ong KG, Dickey EC, Gremes CA. Extreme changes in the electrical resistance of titania nanotubes with hydrogen exposure. Advanced Mate-

Functional Nano and Soft Materials, Soochow University, Suzhou, Jiangsu, China 2 Department of Chemistry, University of Western Ontario, London, ON, Canada

Author details

References

Figure 12. 2D XANES-XEOL plot of TiO2 hierarchical structures annealed at different temperatures.

This technique sheds light on tracking the fabrication of light-emitting devices with spatially separated luminescence.

TiO2 NT can also serve as a substrate for nanoparticle deposition. Noble metal loaded TiO2 NT has been used in photocatalysis [42, 43] because the electron-hole recombination in TiO2 can be effectively retarded with the presence of noble metal, leading to an enhanced photocatalytic efficiency [44]. The role of noble metal nanoparticles in modifying electronic structure of TiO2 is investigated using XEOL-XANES. An example shows the study of Pd nanoparticles (PdNP) loaded TiO2 NT compared with pristine TiO2 NT [19]. PdNP is deposited on TiO2 NT by hydrothermal method, and the morphology is shown in Figure 13a.

Upon coating, the luminescence of TiO2 NT changes. XEOL spectra, recorded with excitation energy above the O K-edge, are shown in Figure 13b. A decrease in intensity and a slight shift

Figure 13. (a) The morphology of TiO2NT after Pd deposition. (b) XEOL of TiO2 NT with and without Pd nanoparticlet coating. (Adapted from Ref. [39]).

of the emission wavelength are observed in Pd-loaded TiO2 NT. The presence of Pd has two roles such as modifies the defect states of TiO2 on the surface and quenches the radiative recombination channel in TiO2 by acting as an electron sink. Because of this, the photocatalysis efficiency can be improved with introducing PdNP.
