Supplementary Information Atomic structure of titania nanosheet with vacancies Megumi Ohwada1,2,*, Koji Kimoto1,*, Teruyasu Mizoguchi3, Yasuo Ebina4& Takayoshi Sasaki4 1 Surface Physics and Structure Unit and 4International Center for Materials Nanoarchitectonics, National Institute for Materials Science, Tsukuba, Ibaraki 305-0044, Japan 2 Department of Applied Chemistry, Kyushu University, Tsukuba, Ibaraki 305-0044, Japan 3 Institute of Industrial Science, The University of Tokyo, Meguro, Tokyo 153-8505, Japan 1. Preparation of TEM specimen; UV light illumination for self-cleaning 2. TEM observation 2.1. Details of Ti vacancy observation 2.2. Details of TEM imaging parameters 2.3. Beam damage during the TEM observation 3. First-principles calculation of Ti vacancy structure models 4. Charge redistribution and its effect in high-resolution TEM image References *Correspondence and requests for materials should be addressed to M.O. or K.K. (emails: ohwada.megumi@nims.go.jp or kimoto.koji@nims.go.jp). 1 1. Preparation of TEM specimen; UV light illumination for self-cleaning Ti0.87O2 nanosheets are synthesized as negatively charged colloidal sheets surrounded by tetrabutylammonium (TBA) ions ((C4H9)4N+), which make it difficult to observe the atomic structures of the nanosheets using transmission electron microscopy (TEM). TEM specimens, which are titania nanosheets on a holey carbon film, are illuminated by ultraviolet (UV) light in air to decompose the TBA ions photocatalytically. Figure S1a,b respectively shows a diffraction pattern and a TEM image of a titania nanosheet without UV light illumination. These data were acquired using a conventional 300 kV transmission electron microscope (Hitachi, HF-3000). The halo in the diffraction pattern and the amorphous contrast of the TEM image indicate that a substantial number of TBA ions remained on the nanosheets. Figure S1c,d respectively shows a TEM image and a diffraction pattern of a titania nanosheet after UV light illumination for 2 h. The amorphous structure on the nanosheet in (d) is almost nonexistent and the intensity of the halo in the diffraction pattern in (c) is weaker than that in (a). The TBA ions on the titania nanosheets are considered to be decomposed by the UV light illumination. The chemical compounds remaining after the decomposition of TBA ions are considered to be H3O+ and/or NH4+, which compensate the charge balance of titania nanosheets1. Oxygen atoms in the crystal lattice of the titania nanosheet are not considered to be desorbed through UV light illumination. 2 Figure S1. Effect of UV light illumination on a single titania nanosheet with tetrabutylammonium (TBA) ions. (a) and (b) show a diffraction pattern and a TEM image from a titania nanosheet without UV light illumination, respectively. (c) and (d) were obtained from a titania nanosheet after UV light illumination for 2 h. 2. TEM observation 2.1. Details of Ti vacancy observation The direct observation of atomic structures using TEM is challenging, particularly for materials sensitive to electron beam, because the required dose rate is proportional to the square of the magnification. Here we describe details of our TEM observation. We used an advanced electron microscope (FEI, Titan-Cubed) equipped with a spherical aberration corrector and a monochromator. The acceleration voltage of the microscope was set to 80 kV to decrease knock-on damage. To improve the resolution and contrast of the TEM images, the spherical aberration was corrected to less than 0.001 mm and the energy spread of the microscope was reduced to 0.1 eV in full width at half maximum. To observe a single titania nanosheet without electron beam 3 damage, we applied a low dose rate of 2.5×104 electrons/nm2/s, which is substantially low compared with those used in previously reported high-resolution TEM observations, for example, 3×106 electrons/nm2/s for BN nanosheets2. We also applied a beam blanking system to reduce the total dose. The defocus of the objective lens was set at an underfocus of about 4 nm. TEM images were recorded using a charge-coupled device (CCD) camera of 1024×1024 pixels (Gatan Inc., UltraScan) with an exposure time of 2 s. Since the microscope was installed on an active vibration-isolation system (Tokkyokiki Corporation) in a room with a panel-cooling air conditioning system (Nihon Spindle Manufacturing Co., Ltd.), a high specimen-stage stability of less than 0.2 nm/min was routinely realized. We acquired ten TEM images at the same focus, and the specimen drift between the ten images was corrected after their acquisition using cross correlation. Then we obtained an image with a high signalto-noise (SN) ratio by summation of the ten images (Fig. S2). An acquired raw image is shown in Fig. S3, which indicates that the SN ratio is rather low without summation. Figure S4 shows Ti vacancies in the same area as Fig. 1a in the main text. Although we summed 28 small images of Ti vacancies to improve the SN ratio for the structural analysis (Fig. 2a), we can confirm the presence of about 60 Ti vacancies as indicated by open rectangles. To improve the SN ratio, we also averaged TEM images on the basis of the projected symmetry of the crystal structure (mm, i.e., two perpendicular mirror symmetries). Since the TEM specimen was carefully prepared, the surface was clean and the structural change reported in our previous study3 cannot be seen in Fig. S2. However, very weak extra spots of the titania nanosheets can be seen, as marked by arrows in the Fourier transform in the inset of Fig. S2. It is possible that the missing oxygen atoms around Ti vacancies (discussed in the main text) are responsible for these spots, although it is difficult to confirm the disordered atomic loss from a Fourier transform. 4 Figure S2. Relatively low magnification TEM image of a titania nanosheet and its Fourier transform (inset). The TEM image is the summation of ten images after specimen drift correction. The square in the TEM image indicates the area shown in Figure 1a in the main text. The spots marked by arrows in the Fourier transform are kinematically forbidden. Figure S3. Acquired raw image before summation. The area is the same as the image in Fig. S2. Note the low SN ratio of the image. 5 Figure S4. High-resolution TEM image showing Ti vacancies. The area is the same as the image in Fig. 1a and the square area in Fig. S2. Rectangles indicate Ti vacancies. Scale bar is 1 nm. 2.2. Details of TEM imaging parameters To observe atomic arrangements using high-resolution TEM, it is indispensable to optimize microscope parameters. Here we describe the TEM imaging parameters and contrast transfer function (CTF). The major parameters are as follows; the third-order spherical aberration coefficient C3 = 0 mm, the chromatic aberration coefficient Cc = 1.5 mm, and energy spread ΔE = 0.1 eV at an acceleration voltage E = 80 kV. We draw a CTF under this condition with a defocus z = 4 nm of underfocus (Fig. S5a). In comparison with non-monochromated condition (CTF and Kc in blue), the present condition with a monochromator (those in pink) provides a higher spatial frequency of the information limit, and it improves the contrast of TEM images, as described in a specialized journal4. Figure S5b shows CTFs with different defocus values under the same condition of Fig. S5a with ΔE = 0.1 eV. We chose 4 nm of underfocus to enhance the contrast made of the lattice spacings of the tinania nanosheet (e.g., d200=1.9 Å, d002=1.5 Å). 6 Figure S5. Contrast transfer functions (CTFs) of the high-resolution TEM imaging. (a) CTF with different energy spreads. The CTF with a monochromator (ΔE = 0.1 eV) is indicated in pink, and that without a monochromator is indicated in blue (ΔE = 0.9 eV). Both the defocus value is set to 4 nm of underfocus. Kc (broken lines) are chromatic envelope functions. (b) CTFs with different values of defocus. 2.3. Beam damage during TEM observation Beam damage such as knock-on damage is critical when observing the intrinsic crystal structure of nanosheets. The threshold energy of the incident beam for the atomic displacement of a titanium atom has been reported to be approximately 400 keV5; therefore, the present lower-voltage (80 kV) TEM is considered to be usable for observing atomic vacancies. Here we experimentally investigate the beam damage in the nanosheets during TEM observations at acceleration voltages of 80 and 300 kV. 7 We first observed the dose dependence at an acceleration voltage of 80 kV. Since we acquired many TEM images under a low-dose condition, we can investigate the effect of beam irradiation with atomic resolution. Figure S6 shows TEM images of the same area at total doses of about (a) 2.5×105 electrons/nm2 and (b) 5×105 electrons/nm2, in which the former is a TEM image obtained at the beginning of the experiment. Rectangles indicate the same Ti vacancy sites in the two images, and the distribution of the Ti vacancies was found to be almost identical. The titanium atom marked by the arrow in image (a) disappeared during the TEM observation; however, the estimated ratio of Ti atoms displaced during the TEM measurement was approximately 1% of the Ti atoms in the crystal lattice. Since the present analysis was performed under this total dose condition, almost all the Ti vacancies observed in this study originally existed in the as-prepared titania nanosheets. Figure S6. TEM image sequences showing effect of electron irradiation on Ti vacancies. The images are obtained at total doses of about (a) 2.5×105 electrons/nm2 and (b) 5×105 electrons/nm2. Rectangles indicate common Ti vacancies. The arrows in the two images show the loss of a titanium atom. We also investigated the beam damage at 300 kV. Figure S7 shows typical irradiation damage of a titania nanosheet observed using a conventional microscope (a 300 kV uncorrected microscope with a cold field emission gun, Hitachi HF-3000) at a dose rate of 2.6×105 electrons/nm2/s, which is low in comparison with the conventional dose rate. Small holes were produced by electron irradiation at the beginning of high-resolution observation, and the sheet was torn within a couple of minutes. Thus, lower-voltage TEM observation is indispensable to investigate the atomic structure of titania nanosheets. 8 Figure S7. High-resolution observation of a titania nanosheet using a 300 kV conventional electron microscope with a cold field emission gun. The nanometer-sized holes marked by arrows were formed after 2 min observation. 3. First-principles calculation of Ti vacancy structure models We examined three structure models with different arrangements of oxygen atoms around Ti vacancies. The optimization of the structure around each Ti vacancy was performed by first-principles plane-wave basis pseudopotential calculations within the generalized gradient approximation (GGA) using the CASTEP code6. Ultrasoft pseudopotentials were used and the plane-wave cutoff energy was taken to be 340 eV. Before the optimization, the theoretical lattice constants of the titania nanosheet were obtained. The calculation was performed using a unit cell of the titania nanosheet (a = 3.83 Å, c = 3.00 Å), in which each nanosheet is separated from adjacent nanosheets by 17 Å. The calculated lattice constants are a = 3.762 Å and c = 3.0258 Å, which are in good agreement with the experimental lattice constants, although the calculation slightly overestimates the lattice constants due to the GGA error. To introduce vacancies, the calculated unit cell was expanded by 5×1×5 and 150-atom supercells were used. In the vacancy calculation, the Monkhorst-Pack k point was set to 2×1×2. All atoms were relaxed until their residual force became smaller than 0.1 eV/Å. During the atomic relaxation, each volume was fixed. In all cases, neutral vacancies were considered. Figure S8 shows the three structures examined: (a) Ti vacancy model, (b) Ti+Oc1+Oc2 vacancy model, and (c) Ti+Oa1+Oa2 vacancy model. Each atomic 9 arrangement was optimized to minimize the residual forces. Here, we compare the optimized structures with the non-optimized ones. The non-optimized structures of the three models (Fig. S8) generated similar TEM simulation images, although the contrasts of oxygen sites around the Ti vacancies depended on the models. When the structures are optimized, the local structures around the Ti vacancies are differently relaxed, resulting in changes in the simulation images. From a total energy calculation, the Ti+Oc1+Oc2 vacancy model was found to be 3.45 eV more stable than the Ti+Oa1+Oa2 vacancy model, indicating that the Ti+Oc1+Oc2 vacancy model is feasible and that the oxygen atoms c1 and c2 tend to be removed during reduction, which is consistent with the quantitative analyses of TEM image intensities (Fig. 3d,e in the main text). Figure S8. The three Ti vacancy structure models before and after optimization and their simulation TEM images. The structures and simulation images on the left are not optimized, whereas those on the right are optimized. (a) Ti vacancy model. (b) Ti+Oc1+Oc2 vacancy model. (c) Ti+Oa1+Oa2 vacancy model. The numbers in the structure drawings indicate those of existing oxygen atoms. The arrows in each optimized structure in a-c indicate the main directions of atomic distortion resulting from structure optimization (large and small arrows indicate titanium and oxygen atoms, respectively). 10 4. Charge redistribution and its effect in high-resolution TEM image A few advanced studies on high-resolution TEM include the effect of charge distribution on image contrast7,8. Here we briefly discuss its effect in our monochromated aberration-corrected TEM observation. The net charge of each atom can be evaluated by the first-principles calculations as shown in Fig. S9. The net charge of titanium atoms in a titania nanosheet was found to be 1.32, and that of oxygen atoms -0.71 or -0.60. Around a Ti+Oc1+Oc2 vacancy, the net charge of titanium atoms ranged from 1.31 to 1.33. That of oxygen atoms was from -0.69 to -0.59. These first-principles calculations indicate that the charge redistribution near the Ti vacancy is not large. This allows us to use the same atomic scattering factors for all the oxygen/titanium atoms in TEM image simulation. Figure S9. Net charge of Ti+Oc1+Oc2 vacancy model. Numbers in the structure drawing indicate calculated values of the net charge around the vacancy. Although the charge states of Ti vacancy models were obtained by the first-principles calculations, the high-resolution TEM image simulation without charge distribution parameters is still usable as remarked below. The electron scattering factors for oxygen (O0 and O1-) and titanium atoms (Ti0 and Ti4+) listed in the literature9 are plotted in Fig. S10. The atomic scattering factors of neutral and ionized atoms differ in 11 the low-angle range (sinθ/λ < 0.15-0.20 Å-1). We draw the CTF under the present TEM condition in Fig. S10. The present high-resolution TEM imaging does not include the low-angle scattering (sinθ/λ < 0.15 Å-1) so much because of the small defocus and the small spherical aberration in the monochromated aberration-corrected TEM. This implies that the simulation with the parameter of charge distributions may not induce a significant difference from the simulation with neutral atoms. It would be possible that the simulation including ionic states more reproduce the contrast of the TEM image. Figure S10. Atomic scattering factors for titanium and oxygen atoms. CTF under the present TEM condition (Fig. S5a,b) is displayed with the same scale of the lattice spacing. References 1. 2. 3. 4. 5. 6. 7. 8. Sasaki, T. et al. Titania nanostructured films derived from a titania nanosheet/polycation multilayer assembly via heat treatment and UV irradiation. Chem. Mater. 14, 3524-3530 (2002). Alem, N. et al. 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Nature Mater. 10, 209-215 (2011). 12 9. Colliex, C. et al. International Table for Crystallography 2nd edn, Vol. C (eds Wilson, A. J. C. & Prince, E.) 259-283 (Kluwer Academic, London, 1999). 13