Calcination-Induced Phase Transformation and Accompanying Optical

Nov 15, 2010 - Calcination-Induced Phase Transformation and Accompanying Optical Luminescence of. TiO2 Nanotubes: An X-ray Absorption Near-Edge ...
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J. Phys. Chem. C 2010, 114, 21353–21359

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Calcination-Induced Phase Transformation and Accompanying Optical Luminescence of TiO2 Nanotubes: An X-ray Absorption Near-Edge Structures and X-ray Excited Optical Luminescence Study Lijia Liu, Jeffrey Chan, and Tsun-Kong Sham* Department of Chemistry, UniVersity of Western Ontario, 1151 Richmond Street, London, Ontario N6A5B7, Canada ReceiVed: September 29, 2010; ReVised Manuscript ReceiVed: October 27, 2010

Vertically grown TiO2-nanotubes (TiO2-NT) on a Ti metal substrate synthesized by electrochemical anodization followed by calcination have been studied using X-ray absorption near-edge structures (XANES) and X-ray excited optical luminescence (XEOL). It is found that the TiO2-NT system undergoes a series of phase transformations from amorphous (as-prepared) to anatase (400 °C) to rutile (>600 °C); the phase and morphology transformation is accompanied by unusual light-emitting properties, which are strongly dependent upon the crystal phase and morphology controlled by calcination temperatures, i.e. the as-prepared TiO2-NT exhibits no luminescence, whereas the anatase phase exhibits green luminescence, and an intense near-IR emission dominates in the rutile phase. The implications of these observations are discussed. 1. Introduction Titanium dioxide nanotubes (TiO2-NT) have attracted great interest due to their unique electronic and optical properties, which can be applied in photocatalysis, biosensor, and photochromic devices, among other applications. There are several well-established synthesis techniques for producing TiO2-NT with controlled morphologies and in large quantities, such as sol-gel,1,2 hydrothermal,3,4 and electrochemical synthesis.5-8 Among these methods, electrochemical anodization is of particular interest in that it produces TiO2-NT that are directly attached to the Ti foil. This TiO2-NT-on-Ti structure not only provides a large surface area but also maintains good contact with the metallic foil, making it an excellent substrate material for practical biomedical or photoelectrochemical applications.9-12 Two common crystal phases of TiO2 are anatase and rutile, which are composed of connecting, distorted Ti-O octahedral units. Anatase is more bioactive and robust for catalysis purposes, while rutile is one of the natural minerals and can be used for electronic devices due to its high dielectric constant and thermodynamic stability. The as-synthesized TiO2 nanostructures are usually amorphous and can crystallize into different phases by adjusting the calcination temperature. It has been observed that uncalcinated amorphous TiO2 gradually crystallize into anatase phase after calcination under moderate temperatures (200-600 °C) and that higher temperature usually leads to better crystal structure (i.e., 500 °C to ∼600 °C).13,14 However, increasing temperature will induce a phase transformation from anatase to rutile irreversibly, and the transformation temperature covers a wide range from as low as 400 °C to as high as 1200 °C.15 Extensive studies have been carried on the kinetics of anatase-to-rutile phase transformation in TiO2 particles and thin films and found that the process is strongly dependent on the crystal size, interface and purity, etc.16-18 Though anatase is metastable in the bulk, it has been reported that the downsizing of TiO2 to nanoscale and addition of dopant will increase the thermal stability of anatase and retard the * Author for correspondence. E-mail: [email protected].

anatase-to-rutile transformation.13,19-21 More often than not, TiO2 exists in a mixed phase of amorphous-anatase or anatase-rutile, and the commercially available mixed-phase TiO2 powder (e.g., P25, which is composed of 25% rutile and 75% anatase) has been used in recent years for its comparable performance as pure anatase but is less expensive than pure anatase for dyesensitized solar cell applications.22 Meanwhile, it has been reported that the mixed-phase TiO2 also provides unique charge transfer pathways that do not exist in pure-phase TiO2.23 The difference in the structures of anatase and rutile also leads to interesting light-emitting properties of TiO2 upon UV or X-ray excitation. Luminescence from nanostructured TiO2 and TiO2 thin film has been observed, and the energy of the luminescent band is strongly dependent on the crystal phase.24-26 Both anatase and rutile have indirect band gaps, 3.2 and 3.0 eV, respectively; thus, the band gap emission is rarely observed. Instead, the commonly observed luminescence from anatase (nanocrystals in most cases) is in the visible green region (∼550 nm), and the luminescence is highly sensitive to the surface properties. Rutile, on the other hand, has no visible luminescence but shows a near-IR (NIR) emission (∼800 nm) and is less surface sensitive.24,27,28 Since most of the luminescence studies were carried out using a photoluminescence technique with fixed excitation energy from a UV-visible source (laser or discharge lamp), only the electronic transitions from valence electrons to the conduction band are excited. In this paper, we report a systematic study of the light emission behavior from TiO2-NT and its correlation with phase and electronic structure using synchrotron radiation, a tunable X-ray source. TiO2-NT studied here was synthesized using electrochemical anodization followed by calcination to produce TiO2 of different crystal phases. The phase transformation and the light-emitting properties induced by calcination temperatures were tracked using X-ray absorption near-edge structure (XANES) and X-ray excited optical luminescence (XEOL). XANES refers to the X-ray absorption coefficient in the nearedge region (∼50 eV above the absorption threshold) of a particular core level of an element of interest in the material. By exciting the core electron of the element in a chemical

10.1021/jp1093355  2010 American Chemical Society Published on Web 11/15/2010

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environment to the previously unoccupied electronic states with a tunable synchrotron light source, one can probe the local symmetry and occupation of these states of which the characteristics are governed by the local chemistry. In the case of TiO2, the Ti L3,2-edge probes electron transitions from Ti 2p to the conduction band of Ti 3d character (dipole selection rules), and O K-edge probes the O 1s to 2p transition. Thus, the edge jump and the spectral features (resonance) above the edge provide the information of the unoccupied density of states. Amorphous and crystalline TiO2 (anatase and rutile) differ in d-orbital splitting due to different degrees of distortion of the Ti-O octahedron and long-range order; thus, XANES is a powerful tool to distinguish different crystal phases among amorphous, anatase, and rutile. XANES spectra can be collected in (1) total electron yield (TEY), which measures the total yield of photoelectrons, Auger electrons, and secondary electrons (dominant) upon X-ray excitation, providing the information from sample surfaces of a few nanometers; and (2) X-ray fluorescence yield (FLY), which measures the X-ray fluorescence emission during core hole decay, providing bulk sensitive information (hundreds of nanometers to a few micrometers, depending on the element of interest and excitation energy). Meanwhile, X-ray excited optical luminescence (XEOL), an X-ray photon in, optical photon out technique,29 is used to monitor the optical luminescence (UV-visible-NIR) excited at a desired excitation photon energy, which can excite a particular core electron of a given element to bound, quasi-bound, and continuum states, providing element and site specificity. The energy thus absorbed is, in part, transferred to optical de-excitation channels, resulting in optical luminescence. The extent of this conversion of X-ray energy to optical photons depends on the excitation channel and the nature of the material, e.g. crystallinity, morphology, size, and proximity effects (interface). XEOL can also be used to track XANES using photoluminescence yield (PLY), which is performed by collecting XEOL concomitantly while scanning the energy across the absorption edge. Such PLY/XANES can thus be used to reveal the element or the site that is responsible for the luminescence. Moreover, in cases of multichannel light emissions from the sample, PLY can be recorded in selected wavelength ranges (e.g., within a given energy window for a particular emission channel). 2. Experimental Section TiO2-NT was prepared by anodization of Ti foil (0.1 mm thick, Goodfellow) using a two-electrode electrochemical setup, comprising a homemade electrochemical cell and a DC power supply (Hewlett-Packard, 6209B). A Pt wire was used as the cathode, and the Ti foil, as the anode.30 The electrolyte was 0.5 wt % HF and glycerol in a volumetric ratio of 1:9, and pH ≈ 2. The potential was applied from open-circuit potential to 16 V with a sweep rate of 1 V s-1, and then held at 16 V at room temperature for 6 h. Uniform self-organized TiO2-NT was formed vertically on the Ti substrate. The as-made TiO2-NT was divided into seven pieces, one of which was kept as a reference (as-made), and the rest were calcinated at 400, 500, 600, 650, 700, and 800 °C for 2 h, respectively, under ambient conditions, henceforth denoted as NT-400, NT-500, NT-650, NT-700, and NT-800. Scanning electron microscopy (SEM) was used for morphologic characterization. Since the nanotubes are perpendicular to the substrate, focused ion beam (FIB) integrated SEM (LEO 1540XB, FIB/SEM) was used for taking the crosssection image. Synchrotron measurements were conducted at the Canadian Light Source (CLS), located on the campus of the University

Liu et al. of Saskatchewan. CLS is a 2.9 GeV third-generation light source operating with 250 mA injection current for these measurements. The Ti L3,2-edge and the O K-edge were measured at the highresolution, undulator-based, spherical grating monochromator (SGM) beamline.31 The absorption spectra were recorded in the mode of total electron yield (TEY) using specimen current, and X-ray fluorescence yield (FLY) using a channel plate detector, which are surface sensitive and bulk sensitive, respectively. XEOL spectra were collected using a dispersive spectrometer (QE65000, Ocean Optics). PLYs were measured where applicable by collecting total (zero order, 200-950 nm) and wavelength-selected luminescence (e.g., visible green and NIR, see below) as the excitation photon energy tuned across the absorption edge. All spectra were normalized to the incident photon flux, which was monitored with a refreshed Au mesh. 3. Results and Discussion 3.1. Morphology. The SEM image of the as-made TiO2NT (top-view) is shown in Figure 1a. The nanotubes are perpendicular to the substrate with an average diameter of ∼50 nm. The side view (inset of Figure 1a) shows that the length of the nanotubes is around 400 nm. Selected SEM images of calcinated TiO2-NT at 500, 600, 650, 700, and 800 °C are shown in Figure 1 b-f. It can be seen that the nanotubular structure remains after calcination at 500 °C, but the formation of surface cracks is observed after calcination at 600 °C. A noticeable deformation of the nanotube structures is observed after calcination at 650 °C, in which aggregation of nanotubes and formation of surface cracks become apparent. It should also be noted that the size of the tube becomes smaller as the temperature increases. A complete deformation of nanotube structures is seen from 700 °C calcinated TiO2, and the surface is dominated by fused columns of microstructures. The 800 °C calcination results in further aggregation of the fused columns into layered structures. This observation indicates that the temperature of calcination is important to the nanotube size as well as to the morphology. As the aggregation has been observed from TiO2 powders during calcination-induced phase transformation,17 changes in TiO2-NT morphology under elevated calcination temperatures in our case are also due to phase transformation. We shall get into this point later. 3.2. XANES of TiO2-NT. Figure 2 shows the Ti L3,2-edge XANES of the as-made and the calcinated TiO2-NT in comparison with TiO2 (commercial, from Sigma-Aldrich) of anatase and rutile. The spectra shown in Figure 2a were recorded in TEY, which probes the sample surface of a few nanometers. Characteristic features of TiO2 can be observed which arise from the transitions of Ti 2p electrons to unoccupied 3d electronic states. More specifically, peaks a, b1, and b2 belong to L3-edge (2p3/2f3d), and peaks c1 and c2 belong to L2-edge (2p1/2f3d). Both edges have two groups of peaks, which can be understood as due to the crystal field splitting of the d orbitals into t2 g and eg under Oh symmetry followed by local distortion resulting in a local symmetry of D2d and D2h for anatase and rutile, respectively. To assign these peaks, molecular orbital consideration is commonly applied in the case of 3d transition metal oxide.32 Peaks a and c1 belong to the 2pft2g transition while peaks b1, b2, and c2 are from 2pfeg transition.33 Let us first look at the two standard samples: TiO2 of anatase and rutile phases. It can be seen that the most significant differences are (1) the intensity ratio of peak b1 and b2; i.e. b1 is more intense than b2 in anatase, but in rutile this is reversed. (2) In the separation between peak c1 and c2 the two peaks are separated by 2.0 eV in anatase but 2.3 eV in rutile. The lowering of the Oh symmetry to D2d and D2h leads to the 2pfeg transition further

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Figure 1. SEM images of (a) as-made TiO2-NT, (b) NT-500, (c) NT-600, (d) NT-650, (e) NT-700, and (f) NT-800; side view of as-made TiO2-NT is shown in the inset of (a).

splitting into peak b1 and b2 at the L3-edge. The eg state is more sensitive to the variation in symmetry, and thus, anatase and rutile phases can be tracked by the relative ratio of peaks b1 and b2.34 At the L2-edge, all features are broader due to lifetime broadening as well as additional L3-edge features running underneath, such that the splitting of peak c2 is no longer seen but the difference in the crystal phases can still be discerned by the intervals between c1 and c2. The separation between c1 and c2 that is larger in rutile than in anatase is correlated with the interval between the t2g and eg states in the two crystal phases. On the basis of these established spectral features, we can track the evolution of crystal phases under different calcination temperatures. We will focus on the TEY spectra. The broad peaks of weak intensities in the as-made TiO2-NT indicate chemical inhomogeneity often associated with amorphous structure. However, after calcination at 400 °C, the spectrum shows well-resolved features of the anatase phase, as can be immediately identified by the b1 b2 intensity ratio, which persists at 500 and 600 °C. Noticeable changes in the spectrum are observed for NT-650, in which peaks b1 and b2 are of similar intensity, and peak c2 shifts slightly to higher energy, widening the separation between c1 and c2. Both observations indicate that the anatase phase is converting to rutile. Further increase of the calcination temperature leads to a complete reversal of the b1 and b2 intensity and the widening of the c1-c2 separation to that characteristic of rutile. Table 1 lists the contribution of anatase and rutile phases in calcinated TiO2-NT using a linear combination fitting by considering commercial anatase and rutile as pure-phase crystals. It can be seen that the anatase phase is

best crystallized at 600 °C calcination, followed by a sudden transformation to the rutile-dominated phase at higher temperature. The phase transformation to rutile takes place with calcination temperature at 650 °C. The significant morphology change observed in SEM is also in agreement with the spectroscopic data. As anatase and rutile differ in density (3.84 g/cm3 and 4.26 g/cm3, respectively) and Ti and O atoms are more closely packed in the latter, the structure of nanotubes no longer remains during transforming to rutile. As shown in the SEM image, the nanotubes gradually collapse with the formation of surface cracks and are finally replaced by dense microstructures (Figure 1e,f). The shrinking of the nanotube in diameter as the temperature increases is different from what has been observed from TiO2 nanoparticles, as in the latter case, one usually finds an increase in size during phase transformation.16,17 The anatase-to-rutile transformation process in nanocrystals has been considered as the creation of nucleation sites by coarsening of anatase and the further rutile crystal growth. The increase in size is due to agglomeration of nanocrystals to reach the critical nuclei size of rutile.20 In our case, the potential nucleation sites for rutile growth are likely created at the weak joints of nanotubes (e.g., coarse points at the side walls of the tubes, nanotube-foil interfaces) by consumption of surrounding anatase, which is similar to the phase transformation of TiO2 film,18 resulting in the decrease in diameter of the nanotubes, and eventually, the agglomeration of the as-formed rutile leads to a layered microstructure. Now let us take a look at the FLY spectra of the TiO2-NT, which are shown in Figure 2b. All spectra display features similar to those of TEY, although the peaks are broadened due

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Figure 3. O K-edge TEY of TiO2-NT (as-made, amorphous), NT400 (∼anatase), NT-650 (mixed phase), and NT-800 (∼rutile).

Figure 2. Ti L3,2-edge XANES of as-made and calcinated TiO2-NT in comparison with anatase and rutile standard powder recorded in (a) TEY and (b) FLY. The dotted vertical lines mark the c1 and c2 position for the as-made and anatase phase of the specimen.

to self-absorption. As in the TEY XANES, the gradual decrease of the ratio of peaks b1 and b2 and an increase of the interval between peaks c1 and c2 are also observed along with the increase of calcination temperature. The temperature at which the spectra change from anatase-dominated to rutile-dominated is ∼650 °C. A broad pre-edge shoulder is clearly observed at ∼455 eV, which is attributed to the metallic Ti substrate, as the probing depth at the Ti L-edge is larger (∼0.1 µm) in the case of FLY than TEY (a few nanometers).35 The substrate contribution is clearly seen in the as-made TiO2-NT and then becomes less noticeable as the temperature increases and disappears from NT-650 and above. The decrease of the Ti foil contribution is due to (1) the oxidation of the interface between TiO2-NT (calcination is done in the ambient atmosphere) and Ti foil, (2) the shrinking of the size of the tube, and (3) the collapse of the nanotube structures and the condensation of the volume due to the phase transformation (rutile has a higher density), which is in agreement with the SEM observation. The trend of phase transformation can also be observed by looking at the O K-edge spectra. Figure 3 shows selected XANES of as-made TiO2-NT, NT-400, NT-650, and NT-800

for comparison. The NT-400 and NT-800 exhibit features identical to those of anatase and rutile, respectively. The first two sharp peaks are from the transition of the O 1s-to-2p component in the TiO2 3d band.32 Due to hybridization between O and Ti, it shows the similarity to the Ti L3,2-edge for tracking different phases based on t2 g-eg splitting shown as a larger interval between peaks a1 and a2 in rutile. The features in the region between 535 and 550 eV are the O 2p weight in the TiO2 3a1 g and 4t1u environment mixed with Ti 4s and 4p states. Differences can also be seen wherein anatase TiO2 shows two peaks and rutile has three peaks (b2 in NT-400 becomes a shoulder of b3 in NT-800). In this region, the NT-650 sample is clearly seen as composed of both anatase and rutile phases. This observation is in total accord with the Ti L3,2-edge results. 3.3. XEOL from Calcinated TiO2-NT. Although the asmade TiO2-NT is not light emitting, the calcinated TiO2 emits light at a wavelength that differs significantly as the calcination temperature varies. Figure 4a shows the XEOL spectra of NT400, NT-500, NT-600, NT-700, and NT-800. The excitation photon energy is 580 eV, which is above both the Ti L3,2-edge and O K-edge. The 400 °C calcinated TiO2-NT emits weak green luminescence, shown as a broad peak centered at 580 nm. The luminescence intensity increases and is slightly blueshifted to 550 nm for NT-500. Meanwhile, there is a weak shoulder appearing in the NIR region of wavelength around 800 nm. The green luminescence shifts further down to 500 nm after calcination at 600 °C, and a strong NIR luminescence is seen at 820 nm with intensity much larger than that of the 500 nm emission. This 820 nm luminescence persists as the calcination temperature increases, while the green emission weakens and becomes unnoticeable. Several interesting features in the visible luminescence region can be observed if we plot the emission spectra in logarithm scale, shown in Figure 4b. Aside from the main broad green emission, a shoulder at ∼345 nm (3.6 eV) is also seen from NT 400 (weak), NT-500, and NT-600, but this emission is not observed from anatase TiO2 crystallites which have a similar onset at ∼400 nm (∼3.1 eV, close to the band gap of anatase). The ∼345 nm emission should be due to the recombination from near band gap exciton due to quantum confinement effect (the size of the tube decreases as temperature increases as seen in the SEM). NT-700 and NT-800 also show weak intensities at

TABLE 1: Linear Combination Fitting Results of TiO2-NT under Different Calcination Temperatures anatase component rutile component

anatase

NT-400

NT-500

NT-600

NT-650

NT-700

NT-800

rutile

1.00 0.00

0.838 0.162

0.930 0.070

0.981 0.019

0.567 0.433

0.110 0.890

0.047 0.953

0.00 1.00

Phase Transformation in Calcinated TiO2 Nanotubes

Figure 4. XEOL spectra collected with excitation photon energy at 580 eV from NT-400, NT-500, NT-600, NT-700, and NT-800; XEOL of anatase and rutile powders are shown as references. (a) Linear plot with near-IR emission shown as a factor of 10 lower than the actual yield; (b) visible emission plotted in log scale and compared with anatase and rutile crystallites. (It should be noted that commercial rutile has some anatase and, hence, the green emission.) The long wavelength onsets are marked with dotted vertical lines; the baseline of the spectra have been shifted for clarity.

the visible region, but the 345 nm shoulder is absent. This observation further confirms that the 345 nm emission belongs to nanosized anatase phase. The onset of the emission spectrum is located at ∼415 nm (3.0 eV), which is close to the band gap of rutile. However, the intensity of green luminescence from rutile TiO2 (NT-700, NT-800) is significantly weaker than that of bulk rutile crystal. Perhaps most interestingly, only calcinated TiO2-NT with rutile structure shows intense NIR emission. The green emission from commercial rutile is most likely due to anatase impurity which is present in these samples. Thus, calcination of nanostructured TiO2-NT results in a rutile phase with unique optical properties which are different from those of its bulk counterpart. From the XANES spectra, we have concluded that different calcination temperatures result in TiO2 of different crystal phases and morphology; in the following we will describe how we reveal the origin of the luminescence from TiO2 using a combination of studies and analysis of wavelength-selected PLY and XANES at the Ti L3,2- and O K-edges. Since the 345 nm emission in NT-500 and NT-600 as well as the visible luminescence in NT-700 and NT-800 are too weak to be separated from the dominant luminescence, we will focus on the broad green emission in the anatase phase and NIR emission in the rutile phase in the following discussion.

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21357 Figure 5 shows a series of PLY spectra of TiO2-NT at Ti L3,2-edge and O K-edge for samples calcinated at temperatures of 400, 500, 600, and 700 °C. Except for the 700 °C sample, all specimens show mainly anatase-phase characteristics, and the green luminescence is clearly visible although the NIR emission intensity increases with increasing annealing temperature. At 700 °C and above, the green luminescence diminishes, the NIR is the only optical channel detectable, and the sample is dominated by the rutile phase. It can be seen from Figure 5 that the luminescence intensities vary as the excitation energy is scanned across both Ti and O edges. It is interesting to note that the variations of two luminescence bands are different, indicating different luminescence mechanisms and origins. Several interesting features are noted from Figure 5. First, as expected, the zero-order PLY exhibits a pattern similar to that of the dominant emission channel. Second, for NT-400, NT-500, and NT-600, the green luminescence is clearly present, and the PLYs at both the Ti and O edges show a positive edge jump, indicating that the X-ray energy is transferred effectively to this optical channel (larger absorption, higher PLY). The trend is clearly seen even from NT-600 in which the contribution from the green band to the total luminescence is fairly small. This kind of luminescence is often associated with the recombination of near band gap or self-trapped exciton. The first assumption can be excluded as the emission energy is well-below the band gap of TiO2 of either phase. We attribute this green luminescence to surface states of anatase (electron traps), which is in agreement with previous observation.25,26,28 This implies that energy transfer to this channel is effective, quenching the formation and decay of near band gap excitons. The blue-shift of the peak center from 580 to 500 nm also indicates that increase of temperature causes modification of the TiO2 surface, and similar results have been observed before from calcinated titania nanoparticles.36 The recombination energy of electron-hole pairs gradually increases as the temperature goes higher, and this is likely due to the band gap widening upon anatase phase crystallization. The disappearance of green emission is associated with this phase transformation and the ultimate collapse of the TiO2-NT. Third, the gradually increased NIR luminescence intensity in NT-600 and NT-700 is caused by the formation of rutile at elevated temperature. The dominant nearIR yield in rutile TiO2-NT shows a partial inversion at the Ti L3,2-edge and a total inversion at the O K-edge. From the PLY spectra, we can see that upon excitation away from the edge (both below and well above the edge) TiO2 emits intense nearIR emission, but just above the edge, the luminescence is partially quenched. This can be attributed to the saturation effect when the luminescence is less likely surface-related, which can be understood as follows: as the secondary process (“reabsorption” of the Auger electrons and fluorescence X-rays) contributes significantly to the energy transfer to the NIR channel, if the penetration depth of the incident photons is shallow, some of the Auger electrons and fluorescence X-rays will escape the surface without contributing to the secondary process, hence, decreasing the quantum yield of the luminescence. Since the attenuation length of the X-rays is very short in these energy regions (e.g., ∼900 nm below and 100 nm above the Ti L3,2-edge), the sampling depth decreases above the edge, reducing the effective energy transfer; at the O K-edge, the penetration depth reduces further, leading to more severe saturation effects. The appearance of NIR emission at the expense of quenching green emission by calcination of TiO2NT is thus due to the switch of the luminescence site from anatase to rutile as a result of elevated temperature; that is that

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Figure 5. PLY-XANES spectra (zero-order and wavelength-selected) at the Ti L3,2-edge (left) and the O K-edge (right) of (a, b) NT-400, (c, d) NT-500, (e, f) NT-600, (g, h) NT-700. The optical window is noted on the plot.

the radiative electron-hole recombination no longer takes place at the surface of TiO2 when anatase TiO2 is converted to rutile. We now return to the observation that the green luminescence is only observed in anatase TiO2-NT with relatively

low calcination temperature. For NT-600, the luminescence is dominated by the NIR emission, although XANES spectra still show anatase character. Thus, we can extrapolate that phase transformation has already taken place at this temper-

Phase Transformation in Calcinated TiO2 Nanotubes ature, albeit the majority, especially the sample surface and near surface region, is still anatase. At this point, there is a competition between two optical decay channels (green and NIR) after photon absorption, and the NIR channel is preferred. The calcination enables the mobility of charge carriers which recombine with defects to emit NIR emission. It confirms that during calcination, the anatase-to-rutile transformation starts from the bottom and the side of the nanotubes rather than the tip, since the near interface region cannot be effectively detected by TEY or FLY. PLY in NIR, however, has a longer attenuation length. This explains why NIR emission, a nano-rutile TiO2-NT characteristic, is observed for NT-600, while the TEY and FLY show a dominant anatase structure. 4. Summary and Conclusion XANES and XEOL studies of electrochemically synthesized TiO2 nanotubes calcinated under elevated calcination temperatures are carried out at the Ti L3,2-edge and the O K-edge. We observed that the TiO2-NT undergoes phase transformation from amorphous to anatase to rutile by increasing the calcination temperature. The phase transformation from anatase to rutile takes place at temperatures between 600 and 700 °C, starting from the interface of the TiO2-NT and Ti substrate toward the tip of the nanotubes. At higher temperatures, the TiO2 nanostructures completely collapse after transformation to the rutile phase. In contrast to conventional TiO2 materials, these nanotube systems exhibit amazing optical properties. We have observed green luminescence from anatase TiO2-NT and very intense NIR emission from rutile TiO2-NT after calcination. The former is highly surface and phase sensitive and is attributed to the luminescence of oxygen vacancies associated with the anatase phase. The NIR emission, on the other hand, is from bulk defects at the center of the rutile phase, and it is a favorable channel in the mixed phase of anatase and rutile. This result confirms a recent study of Li-doped TiO2-NT,37 in which both green and NIR luminescence were observed and the green luminescence vanished in the Li-doped TiO2-NT, which was converted from an anatase phase to a rutile phase by Li doping. Acknowledgment. Research at the University of Western Ontario is supported by NSERC, CRC, CFI, and OIT. The Canadian Light Source is supported by CFI, NSERC, NRC, CIHR, and the University of Saskatchewan. We thank Dr. Todd Simpson of Nanofabrication Laboratory, University of Western Ontario for FIB-SEM imaging. We also thank Dr. Michael Murphy for helping on synchrotron data collection and Tom Regier for technical support at the SGM beamline, Canadian Light Source. References and Notes (1) Zhang, M.; Bando, Y.; Wada, K. J. Mater. Sci. Lett. 2001, 20, 167.

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21359 (2) Kobayashi, S.; Hamasaki, N.; Suzuki, M.; Kimura, M.; Shirai, H.; Hanabusa, K. J. Am. Chem. Soc. 2002, 124, 6550. (3) Chen, Q.; Zhou, W.; Du, G.; Peng, L.-M. AdV. Mater. 2002, 14, 1208. (4) Yao, B. D.; Chan, Y. F.; Zhang, X. Y.; Zhang, W. F.; Yang, Z. Y.; Wang, N. Appl. Phys. Lett. 2003, 82, 281. (5) Zwilling, V.; Aucouturier, M.; Darque-Ceretti, E. Electrochim. Acta 1999, 45, 921. (6) Gong, D.; Grimes, C. A.; Varghese, O. K.; Hu, W.; Singh, R. S.; Chen, Z.; Dickey, E. C. J. Mater. Res. 2001, 16, 3331. (7) Cai, Q.; Paulose, M.; Varghese, O. K.; Grimes, C. A. J. Mater. Res. 2005, 20, 230. (8) Rani, S.; Roy, S. C.; Paulose, M.; Varghese, O. K.; Mor, G. K.; Kim, S.; Yoriya, S.; La Tempa, T. J.; Grimes, C. A. Phys. Chem. Chem. Phys. 2010, 12, 2780. (9) Das, K.; Bose, S.; Bandyopadhyay, A. J. Biomed. Mater. Res. A 2009, 90A, 225. (10) Kodama, A.; Bauer, S.; Komatsu, A.; Asoh, H.; Ono, S.; Schmuki, P. Acta Biomater. 2009, 5, 2322. (11) Wang, H.; Yip, C. T.; Cheung, K. Y.; Djurisic, A. B.; Xie, M. H.; Leung, Y. H.; Chan, W. K. Appl. Phys. Lett. 2006, 89, 023508/1. (12) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (13) Zhang, J.; Li, M.; Feng, Z.; Chen, J.; Li, C. J. Phys. Chem. B 2006, 110, 927. (14) Ghosh, T. B.; Dhabal, S.; Datta, A. K. J. Appl. Phys. 2003, 94, 4577. (15) Shannon, R. D.; Pask, J. A. J. Am. Ceram. Soc. 1965, 48, 391. (16) Gribb, A. A.; Banfield, J. F. Am. Mineral. 1997, 82, 717. (17) Gouma, P. I.; Mills, M. J. J. Am. Ceram. Soc. 2001, 84, 619. (18) Gouma, P. I.; Dutta, P. K.; Mills, M. J. Nanostruct. Mater. 2000, 11, 1231. (19) Zhang, H.; Banfield, J. F. J. Mater. Res. 2000, 15, 437. (20) Kumar, K.-N. P. Scr. Metall. Mater. 1995, 32, 873. (21) Zhang, H.; Banfield, J. F. J. Mater. Chem. 1998, 8, 2073. (22) Yan, M.; Chen, F.; Zhang, J.; Anpo, M. J. Phys. Chem. B 2005, 109, 8673. (23) Li, G.; Gray, K. A. Chem. Phys. 2007, 339, 173. (24) Knorr, F. J.; Zhang, D.; McHale, J. L. Langmuir 2007, 23, 8686. (25) Abazovic, N. D.; Comor, M. I.; Dramicanin, M. D.; Jovanovic, D. J.; Ahrenkiel, S. P.; Nedeljkovic, J. M. J. Phys. Chem. B 2006, 110, 25366. (26) Zhang, Y. X.; Li, G. H.; Jin, Y. X.; Zhang, Y.; Zhang, J.; Zhang, L. D. Chem. Phys. Lett. 2002, 365, 300. (27) Knorr, F. J.; Mercado, C. C.; McHale, J. L. J. Phys. Chem. C 2008, 112, 12786. (28) Shi, J.; Chen, J.; Feng, Z.; Chen, T.; Lian, Y.; Wang, X.; Li, C. J.Phys. Chem. C 2007, 111, 693. (29) Sham, T. K.; Jiang, D. T.; Coulthard, I.; Lorimer, J. W.; Feng, X. H.; Tan, K. H.; Frigo, S. P.; Rosenberg, R. A.; Houghton, D. C.; Bryskiewicz, B. Nature 1993, 363, 331. (30) Fang, H.-T.; Liu, M.; Wang, D.-W.; Sun, T.; Guan, D.-S.; Li, F.; Zhou, J.; Sham, T.-K.; Cheng, H.-M. Nanotechnology 2009, 20, 225701/1. (31) Regier, T.; Paulsen, J.; Wright, G.; Coulthard, I.; Tan, K.; Sham, T. K.; Blyth, R. I. R. AIP Conf. Proc. 2007, 879, 473. (32) Grunes, L. A.; Leapman, R. D.; Wilker, C. N.; Hoffmann, R.; Kunz, A. B. Phys. ReV. B: Condens. Matter 1982, 25, 7157. (33) De Groot, F. M. F.; Fuggle, J. C.; Thole, B. T.; Sawatzky, G. A. Phys. ReV. B: Condens. Matter 1990, 41, 928. (34) De Groot, F. M. F.; Figueiredo, M. O.; Basto, M. J.; Abbate, M.; Petersen, H.; Fuggle, J. C. Phys. Chem. Miner. 1992, 19, 140. (35) X-ray Calculator: http://henke.lbl.gov/optical_constants/atten2.html. (36) Jung, K. Y.; Park, S. B.; Anpo, M. J. Photochem. Photobiol., A 2005, 170, 247. (37) Zhou, J. G.; Fang, H. T.; Maley, J. M.; Murphy, M. W.; Peter Ko, J. Y.; Cutler, J. N.; Sammynaiken, R.; Sham, T. K.; Liu, M.; Li, F. J. Mater. Chem. 2009, 19, 6804.

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