Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (1): 71-76

#### The article information

Hao-qi Tang, Yue Lin, Zheng-wang Cheng, Xue-feng Cui, Bing Wang

Direct View of Cr Atoms Doped in Anatase TiO2(001) Thin Film
Cr掺杂锐钛矿型TiO2（001）薄膜中Cr原子的直接观测
Chinese Journal of Chemical Physics, 2018, 31(1): 71-76

http://dx.doi.org/10.1063/1674-0068/31/cjcp1705103

### Article history

Accepted on: June 6, 2017
Direct View of Cr Atoms Doped in Anatase TiO2(001) Thin Film
Hao-qi Tang, Yue Lin, Zheng-wang Cheng, Xue-feng Cui, Bing Wang
Dated: Received on May 22, 2017; Accepted on June 6, 2017
Hefei National Laboratory for Physical Sciences at the Microscale, Department of Physics, Synergetic Innovation Center of Quantum Information & Quantum Physics and Key Laboratory of Strong-Coupled Quantum Matter Physics (CAS), University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Bing Wang, E-mail:bwang@ustc.edu.cn
Abstract: Imaging the doping elements is critical for understanding the photocatalytic activity of doped TiO2 thin film. But it is still a challenge to characterize the interactions between the dopants and the TiO2 lattice at the atomic level. Here, we use high angle annular dark-field/annular bright-field scanning transmission electron microscope (HAADF/ABF-STEM) combined with electron energy loss spectroscopy (EELS) to directly image the individual Cr atoms doped in anatase TiO2(001) thin film from [100] direction. The Cr dopants, which are clearly imaged through the atomic-resolution EELS mappings while can not be seen by HADDF/ABF-STEM, occupy both the substitutional sites of Ti atoms and the interstitial sites of TiO2 matrix. Most of them preferentially locate at the substitutional sites of Ti atoms. These results provide the direct evidence for the doping structure of Cr-doped A-TiO2 thin film at the atomic level and also prove the EELS mapping is an excellent technique for characterizing the doped materials.
Key words: Anatase TiO2    Cr dopants    Scanning transmission electron microscope    Electron energy loss spectroscopy    Pulsed laser deposition
Ⅰ. INTRODUCTION

As the model system of metal oxides, titanium dioxide (TiO$_2$) has been intensively studied since it is considered as a promising photocatalyst in energy conversion [1-9] and environmental protection [3, 4, 10-13]. Detailed atomic-scale knowledge of the structure and sites of the doping elements is quite of importance for understanding the intrinsic and the functional chemical and physical properties in doped TiO$_2$ [3-5, 8, 12, 14, 15]. It has been reported that Cr atoms as one of the most effective doping elements in the TiO$_2$ matrix for narrowing the bandgap could move the photocatalytic response from UV light to abundant visible light region [16-26]. In the doped TiO$_2$ thin film, the locations of the dopants, which have been experimentally observed by X-ray photoelectron spectroscopy (XPS) [22, 27, 28], scanning tunneling microscopy (STM) [29-31], Raman spectroscopy [32], extended X-ray absorption fine structure (EXAFS) [33, 34] and Rutherford backscattering spectrometry (RBS) [34], play the main decisive roles for its photocatalytic efficiency [9]. Previous first-principles calculations predicted that the interstitial sites and the substitutional sites are all the energetically favorable sites for the Cr atoms [35]. Even so, if the dopants reside at the interstitial sites, the discrete dopant level will pin inside the band gap [35], which not only compromise the effectiveness of the band gap narrowing but also can create the additional trapping and recombination centers [36, 37], and result in degrading the mobility of photogenerated electron-hole pairs and reducing the photo activity of the doped TiO$_2$ [38]. So the doping sites of Cr atoms has thus been considered to be of primal importance to enhance the photocatalytic activity in doped TiO$_2$ thin film. Chang et al. and Shibata et al. have directly imaged the metal atoms and nanoparticles adsorbed on TiO$_2$(110) surface at the atomic-scale [14, 39]. Both of them proved the high angle annular dark-field scanning transmission electron microscope (HADDF-STEM) is an excellent technique for directly imaging individual metal atoms supported by TiO$_2$ surfaces. However, for the doped TiO$_2$, no methods have been performed to directly image the doping atoms to study the interactions between the dopants and the TiO$_2$ matrix by now.

In this work, we use the high angle annular dark-field/annular bright-field scanning transmission electron microscope (HAADF/ABF-STEM) combined with EELS to directly image Cr atoms in Cr-doped anatase TiO$_2$(001) single crystal thin film prepared by pulsed laser deposition (PLD) method. From the atomic-resolution STEM images, the Ti and O atoms of anatase TiO$_2$ can all clearly be elucidated, the positions of the doped Cr atoms locating at the interstitial and the substitutional sites of TiO$_2$ matrix are all well identified through EELS mapping. The reason for the locations of most Cr atoms at the substitutional sites is also discussed.

Ⅱ. EXPERIMENTS

The Cr-doped anatase TiO$_2$(001) (A-TiO$_2$) single crystal thin film was epitaxially grown on 0.7 wt% Nb-doped SrTiO$_3$(001) substrates by pulsed laser deposition (PLD) method with a KrF excimer laser (Coherent, 248 nm, operated at a repetition rate of 4 Hz and a pulse duration of 20 ns with an output power of $\sim$200 mJ/pulse for deposition). The atomic ratios of Cr to Ti evaluated by XPS is about 6:94 [22, 23]. The samples could be transferred between the PLD chamber and STM chamber without breaking UHV conditions. Before the deposition, the SrTiO$_3$(001) substrates were pre-sputtered and pre-annealed for several cycles and checked by STM for making sure the surface terminal at the atomic level. The TEM cross-section specimen was prepared by conventional thinning procedure [40]. The material was vertically cut into slices with thickness about 500 µm and mounted on a TEM sample holder at first. Afterwards, it was mechanically thinned to approximately 5 µm, and then glued on a molybdenum aperture grid. Finally it is ion-milled to electron transparency using Ar$^+$ ions at 3.5 kV and a beam incidence angle of 7°, the rotate speed of sample was kept at 3 r/min. The STEM images were recorded using a probe aberration-corrected JEOL JEM-ARM200F STEM (operated at 200 kV) equipped with a Gatan image filter (GIF, Quantum 965). HAADF-STEM images were collected at semi-convergence angle of 23 mrad. The collection angles of the detectors are 90 and 370 mrad for HAADF-STEM images, while 11 and 22 mrad for ABF-STEM images. All EEL spectra were acquired in diffraction mode with a dispersion of 0.25 eV/channel. The convergence and collection semi-angle for EELS were about 30 and 50 mrad, respectively. All of the images shown in this work were filtered and the background was subtracted using a power law in Gatan DigitalMicrograph software.

Ⅲ. RESULTS AND DISCUSSION

FIG. 1(a) shows the large scale STM image of the Cr-doped anatase TiO$_2$(001) surface at room temperature. The (1$\times$4) surface reconstruction, which was typically formed during growth in ultra-high vacuum (UHV) conditions and is considered to be the representative formed for a relative low level reduction, can be observed with the ridges running along the [100] or [010] directions. The spots on the ridges are the defects which have been proved to be the active sites for the dissociation of methanol [41]. FIG. 1(b) shows an atomic-resolution HAADF-STEM image of the anatase (001) thin film observed from the [100] direction. The large scale HAADF-STEM image of the cross-section specimen consists of the SrTiO$_3$ (STO) substrate, A-TiO$_2$ thin film and epoxy. The thickness of A-TiO$_2$ film is estimated to be about 9.3 nm, indicating a relative slow growth rate of $\sim$1.0 nm/h. On the other hand, both the film and the substrate are all atomically flat and the interface determination of A-TiO$_2$/STO is based on the arrangement of atomic columns in bulk STO and A-TiO$_2$, confirming the formation of a clean and atomically abrupt interface between the two oxides. That means the single crystal A-TiO$_2$ thin film was successfully epitaxial growth on STO substrate. The crystallographic relationship between the A-TiO$_2$ film and the STO substrate is determined by selected area diffraction (SAD) shown in FIG. 1(c). The typical SAD pattern is collected from a region containing both the A-TiO$_2$ film and the STO substrate. It is obvious that the TiO$_2$/STO heterostructure shows a crystallographic orientation relationship of (001)(010)TiO$_2$$|$$|$(001)(010)STO.

 FIG. 1 (a) STM image of Cr-doped anatase TiO2(001) surface acquired at 1.5 V and 10 pA. (b) Atomic-resolution HAADFSTEM image of the anatase (001) thin film observed from [100] direction. The (001) surface and the interface between anatase TiO2 and STO are all atomically flat and can be clearly identified. The thickness of the film is estimated to be about 9.3 nm. (c) The SAD pattern of the TiO2/STO heterostructure at the interface region.

FIG. 2(a) shows the ball-stick model of anatase TiO$_2$ viewed from [100] direction. Two types of atomic columns, which were referred to Ti-O columns and O-only columns, alternately align in the [100] projection. FIG. 2(b) shows a typical HAADF STEM image of the A-TiO$_2$ crystal with the referred atomic structure. For HAADF images, the intensity of the atomic columns scales as approximately $Z^{1.7}$, where $Z$ is the average atomic number [3, 42, 43], therefore the contrast is dominated by the cations with higher atomic number, so the O-only columns cannot be imaged since the signal from the light elements is very weak at the HAADF detection angle [44]. Thus the bright spots in FIG. 2(b) correspond to the atomic columns of Ti-O, while the O-only columns appears dark in between the Ti-O columns. In order to identify the O-only columns, we performed the ABF-STEM measurements of the same area as FIG. 2(b). In ABF-STEM image of FIG. 2(c), both light and heavy atom columns are visible simultaneously, and it is an effective means of visualizing light elements [45]. In previous studies of SrTiO$_3$ [46, 47] and rutile TiO$_2$ [45, 47], the O columns are visible as mid-tone dark spots. Similarly the mid-tone dark spots marked by red arrows could be attributed to the O-only columns, which just neighbor to the Ti-O columns marked by the blue arrows.

 FIG. 2 (a) The ball-stick model of anatase TiO2 single crystal viewed from the [100] direction. The TiO2 unit cell is illustrated by a gray regular octahedron. High resolution HAADF (b) and ABF (c) STEM images of A-TiO2(001) thin film with (100) plane projected ball-stick model in the same area. The blue balls represent the sites of Ti atoms while the red balls indicate the location of O atoms.

FIG. 3 shows the EEL spectra of the A-TiO$_2$ film after background subtraction. Cr L-edge is visible in addition to Ti L-edge and O K-edge in the survey spectrum of FIG. 3(a). The corresponding high resolution spectra are shown in FIG. 3(b)-(d). As shown in FIG. 3(b), the Ti L-edge white lines are composed of two sets of local maxima energy ranges, 455-462 eV for L$_3$ and 462-470 eV for L$_2$, arising from transitions of Ti 2p$_{3/2}$ (peaks A1 and B1) and Ti 2p$_{1/2}$ (peaks A2 and B2) core levels into empty Ti 3d states [48], resulting from the core-hole spin-orbit splitting of the 2p levels [49]. The area ratio of L$_3$/L$_2$ is estimated to be 0.79, which is in agreement with the reported experimental value of 0.8 for Ti$^{4+}$ [48, 50]. L$_2$ (and L$_3$) edges are further split into two peaks due to the crystal field interaction, where Ti-O octahedron splits the degenerate Ti 3d states into two states of t$_{2\textrm{g}}$ and e$_\textrm{g}$ [51, 52]. The energy separation of peaks A1 and A2 is 5.3 eV, indicates the oxidation state of Ti is +4 again [51]. On the other hand, the intensity of peak B is stronger than peak A, in agreement with the L$_2$ and L$_3$ shape of anatase phase, as the situation for rutile is inversed [51, 53]. Furthermore, due to the distortion of TiO$_6$ octahedron and their assembly way, the local point-group symmetry around the Ti atom in anatase and rutile is lowered from O$_\textrm{h}$ to D$_{2\textrm{d}}$ and D$_{2\textrm{h}}$ [54]. This will result in the further splitting of L$_3$-e$_\textrm{g}$ (B1) peak into two [55], for the situation as we show (indicated by the black arrows), the low-energy side peak dominates over the high-energy side one corresponds to anatase phase [56], which is consistent with ahead discussion.

 FIG. 3 EEL spectra of the Cr-doped A-TiO2 film. (a) Survey, (b) Ti L-edge, (c) Cr L-edge, (d) O K-edge. The EEL spectra were processed using a power law for background subtraction in Gatan DigitalMicrograph software.

For Cr L-edge as shown in FIG. 3(c), it is also split into peak L$_3$ and peak L$_2$, which can be attributed to the excitation of the spin-orbit splitting levels of Cr 2p$_{3/2}$ and Cr 2p$_{1/2}$ subshells to unoccupied Cr 3d levels [50]. The estimated area ratio of L$_3$/L$_2$ is 1.62, Sparrow et al. reported that the ratio is 1.6 for Cr$_2$O$_3$ and 1.4 for CrO$_2$ [57]. Daulton et al. gave the value for Cr$^{2+}$ larger than 2.0 [58, 59]. Therefore the oxidation state of Cr dopant in the Cr-doped A-TiO$_2$ should be +3, that is, substitutional Cr, similar to the XPS [22] and EXAFS [33] results.

FIG. 3(d) shows the EEL spectrum of O K-edge. Two peaks A and B, where peak B is broader than A, can be attributed to the crystal-field splitting of the t$_{2\textrm{g}}$ and e$_\textrm{g}$ orbitals [60, 61], similar to the result of Ti L-edge. The separation between them is about 2.6 eV, consistent with the result in TiO$_2$, but differs from Cr$_2$O$_3$ (2.3 eV) [51, 62], informing that O atoms mainly bond with Ti atoms, as the atomic concentration of Cr is just 6% relative to Ti. The features at loss energies greater than 535 eV (peaks C, D, E) are assigned to the hybridization of oxygen 2p state with metal 4s and 4p states, and the feature's line shapes reflect the octahedral symmetry exist [60-63].

As the atomic numbers of dopant Cr ($Z$=24) and host Ti ($Z$=22) are almost the same, they cannot be resolved through the HAADF or ABF-STEM images in FIG. 4. However, the existence of Cr dopants has been proven from the EEL spectra in FIG. 3. In order to clarify the locations of Cr dopants in Cr-doped anatase TiO$_2$ thin film, we further take the EELS mapping [64], which is thought to be a helpful technique for viewing the doping elements, to investigate the locations of Cr dopants in the host A-TiO$_2$. FIG. 4(a) shows a large area HAADF-STEM image of the Cr-doped A-TiO$_2$ thin film, which is the same as FIG. 1(b). The small area marked by yellow rectangle was selected for the EELS mapping. FIG. 4 (b) and (c) give the EELS mappings of Ti and Cr elements, respectively. In FIG. 4(c), Cr signal can be clearly seen. Compared with the Ti EELS mapping in FIG. 4(b), the bright spots in FIG. 4(c), which is acquired simultaneously, are not all at the positions of the Ti-O columns. The spots marked by white arrows are in the Ti-O columns, while the other spots marked by yellow arrows are in the interstitial sites between Ti-O and O-only columns. It is noted that the spots in FIG. 4(c) present different intensities, that means the number of Cr atoms in the corresponding columns is also different. From FIG. 4(c), we may confirm that the Cr dopants mostly prefer to stay at the substitutional sites of Ti atoms.

 FIG. 4 A large area HAADF-STEM image (a), EELS mapping of Ti (b) and Cr (c) elements in Cr-doped A-TiO2 film.
Ⅳ. CONCLUSION

The present study demonstrates that atomic structure of doping elements can not be directly distinguished from the HADDF/ABF-STEM images, but can be imaged through the EELS mapping. Our observations give the direct evidence for the doping structure of Cr atoms in Cr-doped A-TiO$_2$ thin film. The foreign Cr atoms mainly locate at the substitutional sites of lattice Ti atoms, while others locate at the interstitial sites. These results not only present an atomic-scale cornerstone for complex doping configuration in doped A-TiO$_2$, but also provide new possibilities for charactering the atomic-scale doping structure and functional chemistry of metal oxides. The ability to directly see the substitutional and interstitial dopants in the host should substantially assist the understanding of doping process and properties of other metal oxides. The HADDF/ABF-STEM and EELS mapping are the powerful method for direct imaging the complex atomic structure of metal oxide materials.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the Ministry of Science and Technology of China (No.2016YFA0200603 and No.2013CB834605), the "Strategic Priority Research Program" of CAS (No.XDB01020100), the National Natural Science Foundation of China (No.91421313, No.21421063, and No.21573207), and Anhui Provincial Natural Science Foundation (1708085MA06). We thank for the help of Dr. Chao Ma, Lei Shi, Ming Zuo, Shun Tan and Prof. Shu-yuan Zhang of Public Experimentation Center, University of Science and Technology of China.

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Cr掺杂锐钛矿型TiO2（001）薄膜中Cr原子的直接观测