Microscopy, 2015, 77–85 doi: 10.1093/jmicro/dfu099 Advance Access Publication Date: 11 November 2014
Article
EELS study of Fe- or Co-doped titania nanosheets Megumi Ohwada1,2,*, Koji Kimoto1,*, Yasuo Ebina3, and Takayoshi Sasaki3 1
Downloaded from http://jmicro.oxfordjournals.org/ at Universite Laval on May 10, 2015
Surface Physics and Structure Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, 2Department of Applied Chemistry, Kyushu University, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and 3International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan *To whom correspondence should be addressed. E-mail: [email protected] (M.O.); [email protected] (K.K.) Received 2 July 2014; Accepted 15 October 2014
Abstract Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets are Fe- and Co-doped titanium oxides, respectively, and they are synthesized by the exfoliation of lepidocrocite-type layered titanates. We have investigated these nanosheets by electron energy-loss spectroscopy (EELS) using a monochromated transmission electron microscope. The energy-loss near-edge structures (ELNESs) of Fe-L and Co-L indicate that Fe3+ and Co2+ ions are substituted in the octahedral sites in each nanosheet. The Ti-L edges of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets correspond to the octahedral coordination of Ti4+ and oxygen atoms as well as an undoped titania nanosheet (Ti0.87O2). On the other hand, the electron transitions from 2p3/2 to 3d eg in Ti-L3 regions are different in each nanosheet. We have also investigated the electron-beam-induced damage of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets. The results indicated that Fe3+ ions in the Ti0.6Fe0.4O2 nanosheets were selectively reduced to Fe2+ ions in the reduction process by electron irradiation. In contrast, the chemical shift of the Ti-L edge of the Ti0.8Co0.2O2 nanosheets indicated that Ti4+ ions were reduced. These results suggest that the substitution of 3d metals in titania nanosheets affects their crystal and electronic structures and material properties such as their longrange atomic configuration and reduction mechanism. Key words: electron energy-loss spectroscopy, energy-loss near-edge structure, monochromated transmission electron microscopy, titanium oxide nanosheet, Fe-, Co-substitution, electron-beam-induced reduction
Introduction Atomic doping is a typical means of obtaining additional properties of materials, and the doping of various elements into nanomaterials has already been explored and studied. The doping of titania nanosheets, which are atomically thin titanium oxides, with transition metals has also been investigated [1–3]. Transition-metal-doped titania nanosheets are synthesized by the same procedure as titania nanosheets, i.e. the exfoliation
of lepidocrocite-type layered titanates doped with transition metals such as Co, Fe and Mn. Transition-metal-doped titania nanosheets are attracting considerable interest owing to their magnetic and optical properties. In addition, they are used as building blocks to construct new materials [4]. Thus, transition-metal-doped nanosheets should be characterized in terms of various aspects such as their electronic and crystal structures. Note that the crystal structures of these
© The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: [email protected]
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Methods Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets were obtained by the exfoliation of K0.8Ti1.73Li0.27O4 [18], K0.8Fe0.8Ti1.2O4 [3,19] and K0.8Co0.4Ti1.6O4, respectively. As they are the same type of lepidocrocite layered titanate of K0.8Ti2−yMyO4 (M = Fe, Co and Li), the resulting nanosheets of Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 are expected to have the same structure, as shown in Fig. 1 [20,21]. These nanosheets were synthesized as negatively charged colloidal crystallites surrounded by positively charged tetrabutylammonium (TBA) ions ((C4H9)4N+). A suspension of the nanosheets was placed on a holey carbon film on a copper grid for TEM experiments. The specimens were illuminated by ultraviolet (UV) light to
Fig. 1. Crystal structure of a basic titania nanosheet. In actual structure, some portions of Ti4+ sites are substituted by lower valence metal ions such as Fe3+ and Co2+ or turn into vacancies to produce a negative charge on the nanosheet. The structures were drawn with VESTA program [22].
photocatalytically decompose the TBA ions surrounding the nanosheets. We used a transmission electron microscope (Titan3, FEI) operated at 80 kV to reduce damage to the specimens because oxygen atoms are easily removed by electron irradiation, i.e. knock-on damage [23,24]. The energy resolution was improved using a monochromator. Low electron dose rates in the range from 103 to 104 electrons (nm−2 s−1) were set in the investigation of both the high-resolution ELNES and the electron dose dependence. The EELS measurements were carried out using an energy filter (Gatan Inc., GIF Quantum ERS). The EEL spectra were acquired in diffraction mode with an entrance aperture of 2.5 mm, which corresponds to a semiangle of 6.5 mrad. The EEL spectra were acquired from areas of about 200 nm diameter. To measure ELNESs at a high energy resolution, the energy dispersion was set to 0.05 eV channel−1 for the Ti-L, Fe-L and Co-L edges. We acquired O-K edges at an energy dispersion of 0.25 eV channel−1. The integration time was set to 20–110 s for each edge. To observe the electron dose dependence of the Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets, we measured their spectra in a wide energy range to include the Ti-L, O-K, Fe- L and Co-L edges at an energy dispersion of 0.25 eV channel−1. The electron dose rate was set to 6.2 × 103 and 1.0 × 104 electrons (nm−2 s−1) for
Downloaded from http://jmicro.oxfordjournals.org/ at Universite Laval on May 10, 2015
nanosheets are basically discussed on the basis of those of the bulk titanates, and direct observations of the nanosheets are difficult using conventional electron microscopy owing to beam damage and the low contrast of their ultrathin structures [5,6]. Electron energy-loss spectroscopy (EELS) in transmission electron microscopy (TEM) is a useful means of revealing both the electronic and atomic structures of transition-metal oxides in specific areas. Various transition-metal oxides have been investigated using TEM–EELS, in which the energyloss near-edge structure (ELNES) provides detailed information about the valence states, bonding states and coordination environments [7–15]. The L edge spectra of transition metals mainly originate from the electron transitions from 2p orbitals to empty 3d or 4s orbitals. The O-K edge, which originates from the electron transition from 1 s orbitals to the hybridized orbitals of oxygen 2p and metal 3d and 4sp orbitals, also contains information on the electronic structures. In addition, TEM–EELS has the advantage of allowing the in situ observation of chemical and structural changes induced by electron irradiation. For example, investigations on the chemical shift and the change in the L3/L2 intensity ratio during electron irradiation have been reported for transition-metal oxides as well as those on structural changes using TEM–EELS [5,16,17]. These results are important for understanding and controlling material properties. EELS measurements to obtain chemical information require high energy resolution. In this study, the high-energyresolution ELNESs of Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets were investigated using monochromated TEM. We studied the coordination and valence states of Ti, Fe and Co atoms in the titania-based nanosheets. We also investigated the changes in these metal-doped titania nanosheets induced by electron irradiation. We discuss the difference in their changes and compare the results with those in our previous report on the reduction of a Ti0.87O2 nanosheet by electronbeam irradiation [5].
Microscopy, 2015, Vol. 64, No. 2
Microscopy, 2015, Vol. 64, No. 2
the Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets, respectively. Each spectrum was collected with an integration time of 100 s.
Results
we aligned all the peaks of L3-t2g to 458.6 eV as shown in Fig. 2a, b and d. The second peaks in the Ti-L3 regions, corresponding to the transition from the 2p3/2 to 3d eg states, show differences in the additional splitting as indicated by arrows in these spectra. The peak in the Ti0.87O2 specimen exhibits a clear splitting of 1.0 eV. In the spectrum of the Ti0.8Co0.2O2 nanosheets, the L3-eg region has a broadened peak with 0.8 eV splitting. In contrast, the corresponding spectrum for Ti0.6Fe0.4O2 has a relatively sharp peak without splitting at 460.8 eV. In the Fe-L edge of the Ti0.6Fe0.4O2 nanosheets, a prepeak can be seen 1.4 eV below the main peak as shown in Fig. 2c. Figure 2e shows that the Co-L edge also has a prepeak, as marked by an arrow, although the splitting is not obvious. These ELNESs are discussed in the following section. We next compared the O-K edges of the Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets. They show similar spectral features with a splitting structure in the lower-energy region around 535–540 eV (A–B) and a major peak in the higher energy region (C–D) as shown in Fig. 3. For comparison, the first peaks of the splitting were set at
Fig. 2. Core-loss spectra of titania-based nanosheets with high energy resolution; (a) Ti-L edge of Ti0.87O2 nanosheets. (b) Ti-L edge and (c) Fe-L edge of Ti0.6Fe0.4O2 nanosheets. (d) Ti-L edge and (e) Co-L edge of Ti0.8Co0.2O2 nanosheets.
Downloaded from http://jmicro.oxfordjournals.org/ at Universite Laval on May 10, 2015
We measured the L edges of Ti, Fe and Co in Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets, respectively. In the investigation of the ELNESs, the energy spread of the incident electrons was
Article
EELS study of Fe- or Co-doped titania nanosheets Megumi Ohwada1,2,*, Koji Kimoto1,*, Yasuo Ebina3, and Takayoshi Sasaki3 1
Downloaded from http://jmicro.oxfordjournals.org/ at Universite Laval on May 10, 2015
Surface Physics and Structure Unit, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, 2Department of Applied Chemistry, Kyushu University, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan, and 3International Center for Materials Nanoarchitectonics, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan *To whom correspondence should be addressed. E-mail: [email protected] (M.O.); [email protected] (K.K.) Received 2 July 2014; Accepted 15 October 2014
Abstract Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets are Fe- and Co-doped titanium oxides, respectively, and they are synthesized by the exfoliation of lepidocrocite-type layered titanates. We have investigated these nanosheets by electron energy-loss spectroscopy (EELS) using a monochromated transmission electron microscope. The energy-loss near-edge structures (ELNESs) of Fe-L and Co-L indicate that Fe3+ and Co2+ ions are substituted in the octahedral sites in each nanosheet. The Ti-L edges of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets correspond to the octahedral coordination of Ti4+ and oxygen atoms as well as an undoped titania nanosheet (Ti0.87O2). On the other hand, the electron transitions from 2p3/2 to 3d eg in Ti-L3 regions are different in each nanosheet. We have also investigated the electron-beam-induced damage of Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets. The results indicated that Fe3+ ions in the Ti0.6Fe0.4O2 nanosheets were selectively reduced to Fe2+ ions in the reduction process by electron irradiation. In contrast, the chemical shift of the Ti-L edge of the Ti0.8Co0.2O2 nanosheets indicated that Ti4+ ions were reduced. These results suggest that the substitution of 3d metals in titania nanosheets affects their crystal and electronic structures and material properties such as their longrange atomic configuration and reduction mechanism. Key words: electron energy-loss spectroscopy, energy-loss near-edge structure, monochromated transmission electron microscopy, titanium oxide nanosheet, Fe-, Co-substitution, electron-beam-induced reduction
Introduction Atomic doping is a typical means of obtaining additional properties of materials, and the doping of various elements into nanomaterials has already been explored and studied. The doping of titania nanosheets, which are atomically thin titanium oxides, with transition metals has also been investigated [1–3]. Transition-metal-doped titania nanosheets are synthesized by the same procedure as titania nanosheets, i.e. the exfoliation
of lepidocrocite-type layered titanates doped with transition metals such as Co, Fe and Mn. Transition-metal-doped titania nanosheets are attracting considerable interest owing to their magnetic and optical properties. In addition, they are used as building blocks to construct new materials [4]. Thus, transition-metal-doped nanosheets should be characterized in terms of various aspects such as their electronic and crystal structures. Note that the crystal structures of these
© The Author 2014. Published by Oxford University Press on behalf of The Japanese Society of Microscopy. All rights reserved. For permissions, please e-mail: [email protected]
77
78
Methods Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets were obtained by the exfoliation of K0.8Ti1.73Li0.27O4 [18], K0.8Fe0.8Ti1.2O4 [3,19] and K0.8Co0.4Ti1.6O4, respectively. As they are the same type of lepidocrocite layered titanate of K0.8Ti2−yMyO4 (M = Fe, Co and Li), the resulting nanosheets of Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 are expected to have the same structure, as shown in Fig. 1 [20,21]. These nanosheets were synthesized as negatively charged colloidal crystallites surrounded by positively charged tetrabutylammonium (TBA) ions ((C4H9)4N+). A suspension of the nanosheets was placed on a holey carbon film on a copper grid for TEM experiments. The specimens were illuminated by ultraviolet (UV) light to
Fig. 1. Crystal structure of a basic titania nanosheet. In actual structure, some portions of Ti4+ sites are substituted by lower valence metal ions such as Fe3+ and Co2+ or turn into vacancies to produce a negative charge on the nanosheet. The structures were drawn with VESTA program [22].
photocatalytically decompose the TBA ions surrounding the nanosheets. We used a transmission electron microscope (Titan3, FEI) operated at 80 kV to reduce damage to the specimens because oxygen atoms are easily removed by electron irradiation, i.e. knock-on damage [23,24]. The energy resolution was improved using a monochromator. Low electron dose rates in the range from 103 to 104 electrons (nm−2 s−1) were set in the investigation of both the high-resolution ELNES and the electron dose dependence. The EELS measurements were carried out using an energy filter (Gatan Inc., GIF Quantum ERS). The EEL spectra were acquired in diffraction mode with an entrance aperture of 2.5 mm, which corresponds to a semiangle of 6.5 mrad. The EEL spectra were acquired from areas of about 200 nm diameter. To measure ELNESs at a high energy resolution, the energy dispersion was set to 0.05 eV channel−1 for the Ti-L, Fe-L and Co-L edges. We acquired O-K edges at an energy dispersion of 0.25 eV channel−1. The integration time was set to 20–110 s for each edge. To observe the electron dose dependence of the Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets, we measured their spectra in a wide energy range to include the Ti-L, O-K, Fe- L and Co-L edges at an energy dispersion of 0.25 eV channel−1. The electron dose rate was set to 6.2 × 103 and 1.0 × 104 electrons (nm−2 s−1) for
Downloaded from http://jmicro.oxfordjournals.org/ at Universite Laval on May 10, 2015
nanosheets are basically discussed on the basis of those of the bulk titanates, and direct observations of the nanosheets are difficult using conventional electron microscopy owing to beam damage and the low contrast of their ultrathin structures [5,6]. Electron energy-loss spectroscopy (EELS) in transmission electron microscopy (TEM) is a useful means of revealing both the electronic and atomic structures of transition-metal oxides in specific areas. Various transition-metal oxides have been investigated using TEM–EELS, in which the energyloss near-edge structure (ELNES) provides detailed information about the valence states, bonding states and coordination environments [7–15]. The L edge spectra of transition metals mainly originate from the electron transitions from 2p orbitals to empty 3d or 4s orbitals. The O-K edge, which originates from the electron transition from 1 s orbitals to the hybridized orbitals of oxygen 2p and metal 3d and 4sp orbitals, also contains information on the electronic structures. In addition, TEM–EELS has the advantage of allowing the in situ observation of chemical and structural changes induced by electron irradiation. For example, investigations on the chemical shift and the change in the L3/L2 intensity ratio during electron irradiation have been reported for transition-metal oxides as well as those on structural changes using TEM–EELS [5,16,17]. These results are important for understanding and controlling material properties. EELS measurements to obtain chemical information require high energy resolution. In this study, the high-energyresolution ELNESs of Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets were investigated using monochromated TEM. We studied the coordination and valence states of Ti, Fe and Co atoms in the titania-based nanosheets. We also investigated the changes in these metal-doped titania nanosheets induced by electron irradiation. We discuss the difference in their changes and compare the results with those in our previous report on the reduction of a Ti0.87O2 nanosheet by electronbeam irradiation [5].
Microscopy, 2015, Vol. 64, No. 2
Microscopy, 2015, Vol. 64, No. 2
the Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets, respectively. Each spectrum was collected with an integration time of 100 s.
Results
we aligned all the peaks of L3-t2g to 458.6 eV as shown in Fig. 2a, b and d. The second peaks in the Ti-L3 regions, corresponding to the transition from the 2p3/2 to 3d eg states, show differences in the additional splitting as indicated by arrows in these spectra. The peak in the Ti0.87O2 specimen exhibits a clear splitting of 1.0 eV. In the spectrum of the Ti0.8Co0.2O2 nanosheets, the L3-eg region has a broadened peak with 0.8 eV splitting. In contrast, the corresponding spectrum for Ti0.6Fe0.4O2 has a relatively sharp peak without splitting at 460.8 eV. In the Fe-L edge of the Ti0.6Fe0.4O2 nanosheets, a prepeak can be seen 1.4 eV below the main peak as shown in Fig. 2c. Figure 2e shows that the Co-L edge also has a prepeak, as marked by an arrow, although the splitting is not obvious. These ELNESs are discussed in the following section. We next compared the O-K edges of the Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets. They show similar spectral features with a splitting structure in the lower-energy region around 535–540 eV (A–B) and a major peak in the higher energy region (C–D) as shown in Fig. 3. For comparison, the first peaks of the splitting were set at
Fig. 2. Core-loss spectra of titania-based nanosheets with high energy resolution; (a) Ti-L edge of Ti0.87O2 nanosheets. (b) Ti-L edge and (c) Fe-L edge of Ti0.6Fe0.4O2 nanosheets. (d) Ti-L edge and (e) Co-L edge of Ti0.8Co0.2O2 nanosheets.
Downloaded from http://jmicro.oxfordjournals.org/ at Universite Laval on May 10, 2015
We measured the L edges of Ti, Fe and Co in Ti0.87O2, Ti0.6Fe0.4O2 and Ti0.8Co0.2O2 nanosheets, respectively. In the investigation of the ELNESs, the energy spread of the incident electrons was
