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Atomic species identification at the (101) anatase surface by simultaneous scanning tunnelling and atomic force microscopy.

Stetsovych O, Todorović M, Shimizu TK, Moreno C, Ryan JW, León CP, Sagisaka K, Palomares E, Matolín V, Fujita D, Perez R, Custance O - Nat Commun (2015)

Bottom Line: Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments.Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects.Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materials.

View Article: PubMed Central - PubMed

Affiliation: 1] National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan [2] Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, Praha 8, Czech Republic.

ABSTRACT
Anatase is a pivotal material in devices for energy-harvesting applications and catalysis. Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments. Here we combine atomic force microscopy (AFM) and scanning tunnelling microscopy (STM), supported by first-principles calculations, for the simultaneous imaging and unambiguous identification of atomic species at the (101) anatase surface. We demonstrate that dynamic AFM-STM operation allows atomic resolution imaging within the material's band gap. Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects. Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materials.

No MeSH data available.


Related in: MedlinePlus

AFM imaging contrast mechanism and common surface defects.(a) Calculated tip–surface interatomic forces over the relevant atomic positions of the TiO2(101) anatase surface and comparison with experimental curves. The insets are simultaneous topographic AFM (Z(Δf), upper panel) and averaged tunnelling current (<It>, lower panel) images of the surface area where the force spectroscopy experiments were performed. The black and red stars mark the acquisition spots. Image dimensions are (1.5 × 1.5) nm2. Acquisition parameters are: fo=160,360 Hz, Δf=−5.0 Hz, A=113.6 Å, K=27.5 N m−1, CPD=−80 mV, VBias=+500 mV. (b) Atomic model used in first-principle calculations, showing the hydroxyl group-terminated sharp TiO2 cluster tip above the the TiO2(101) anatase surface slab. (c) Calculated force spectroscopy curves at the relevant sites of common TiO2(101) surface point defects: a hydrogen defect and a subsurface oxygen vacancy (SSOV)—results for the clean surface sites are provided in dashed line for comparison.
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f3: AFM imaging contrast mechanism and common surface defects.(a) Calculated tip–surface interatomic forces over the relevant atomic positions of the TiO2(101) anatase surface and comparison with experimental curves. The insets are simultaneous topographic AFM (Z(Δf), upper panel) and averaged tunnelling current (<It>, lower panel) images of the surface area where the force spectroscopy experiments were performed. The black and red stars mark the acquisition spots. Image dimensions are (1.5 × 1.5) nm2. Acquisition parameters are: fo=160,360 Hz, Δf=−5.0 Hz, A=113.6 Å, K=27.5 N m−1, CPD=−80 mV, VBias=+500 mV. (b) Atomic model used in first-principle calculations, showing the hydroxyl group-terminated sharp TiO2 cluster tip above the the TiO2(101) anatase surface slab. (c) Calculated force spectroscopy curves at the relevant sites of common TiO2(101) surface point defects: a hydrogen defect and a subsurface oxygen vacancy (SSOV)—results for the clean surface sites are provided in dashed line for comparison.

Mentions: The AFM imaging mechanism on the TiO2(101) anatase surface can be further clarified with the aid of first-principles calculations. Determining a suitable atomic arrangement that qualitatively describes the forefront part of the probe is crucial for the correct interpretation of AFM data363738394041. To model the tip apex, we have chosen small TiO2 clusters terminated by a hydroxyl group, which were found to successfully describe weak tip–surface interatomic forces on TiO2(110) rutile39 and could have been easily formed during the probe conditioning prior to starting the measurements (see Methods). Calculated tip–surface interatomic forces obtained on approaching the probe model over relevant atomic positions of the TiO2(101) anatase surface were compared with the experimental counterparts. Both the magnitude of the force minima and the overall shape of the curves were evaluated for different relative orientations of the tip cluster model, exposing blunt or sharp cluster edges towards the surface, while maintaining the hydroxyl termination. Figure 3a summarises the calculated tip–surface interatomic forces obtained with the sharp probe orientation that best reproduces experimental data—depicted in Fig. 3b—and displays the comparison with a typical set of experimental short-range force curves.


Atomic species identification at the (101) anatase surface by simultaneous scanning tunnelling and atomic force microscopy.

Stetsovych O, Todorović M, Shimizu TK, Moreno C, Ryan JW, León CP, Sagisaka K, Palomares E, Matolín V, Fujita D, Perez R, Custance O - Nat Commun (2015)

AFM imaging contrast mechanism and common surface defects.(a) Calculated tip–surface interatomic forces over the relevant atomic positions of the TiO2(101) anatase surface and comparison with experimental curves. The insets are simultaneous topographic AFM (Z(Δf), upper panel) and averaged tunnelling current (<It>, lower panel) images of the surface area where the force spectroscopy experiments were performed. The black and red stars mark the acquisition spots. Image dimensions are (1.5 × 1.5) nm2. Acquisition parameters are: fo=160,360 Hz, Δf=−5.0 Hz, A=113.6 Å, K=27.5 N m−1, CPD=−80 mV, VBias=+500 mV. (b) Atomic model used in first-principle calculations, showing the hydroxyl group-terminated sharp TiO2 cluster tip above the the TiO2(101) anatase surface slab. (c) Calculated force spectroscopy curves at the relevant sites of common TiO2(101) surface point defects: a hydrogen defect and a subsurface oxygen vacancy (SSOV)—results for the clean surface sites are provided in dashed line for comparison.
© Copyright Policy - open-access
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC4491188&req=5

f3: AFM imaging contrast mechanism and common surface defects.(a) Calculated tip–surface interatomic forces over the relevant atomic positions of the TiO2(101) anatase surface and comparison with experimental curves. The insets are simultaneous topographic AFM (Z(Δf), upper panel) and averaged tunnelling current (<It>, lower panel) images of the surface area where the force spectroscopy experiments were performed. The black and red stars mark the acquisition spots. Image dimensions are (1.5 × 1.5) nm2. Acquisition parameters are: fo=160,360 Hz, Δf=−5.0 Hz, A=113.6 Å, K=27.5 N m−1, CPD=−80 mV, VBias=+500 mV. (b) Atomic model used in first-principle calculations, showing the hydroxyl group-terminated sharp TiO2 cluster tip above the the TiO2(101) anatase surface slab. (c) Calculated force spectroscopy curves at the relevant sites of common TiO2(101) surface point defects: a hydrogen defect and a subsurface oxygen vacancy (SSOV)—results for the clean surface sites are provided in dashed line for comparison.
Mentions: The AFM imaging mechanism on the TiO2(101) anatase surface can be further clarified with the aid of first-principles calculations. Determining a suitable atomic arrangement that qualitatively describes the forefront part of the probe is crucial for the correct interpretation of AFM data363738394041. To model the tip apex, we have chosen small TiO2 clusters terminated by a hydroxyl group, which were found to successfully describe weak tip–surface interatomic forces on TiO2(110) rutile39 and could have been easily formed during the probe conditioning prior to starting the measurements (see Methods). Calculated tip–surface interatomic forces obtained on approaching the probe model over relevant atomic positions of the TiO2(101) anatase surface were compared with the experimental counterparts. Both the magnitude of the force minima and the overall shape of the curves were evaluated for different relative orientations of the tip cluster model, exposing blunt or sharp cluster edges towards the surface, while maintaining the hydroxyl termination. Figure 3a summarises the calculated tip–surface interatomic forces obtained with the sharp probe orientation that best reproduces experimental data—depicted in Fig. 3b—and displays the comparison with a typical set of experimental short-range force curves.

Bottom Line: Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments.Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects.Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materials.

View Article: PubMed Central - PubMed

Affiliation: 1] National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan [2] Charles University, Faculty of Mathematics and Physics, V Holešovičkách 2, Praha 8, Czech Republic.

ABSTRACT
Anatase is a pivotal material in devices for energy-harvesting applications and catalysis. Methods for the accurate characterization of this reducible oxide at the atomic scale are critical in the exploration of outstanding properties for technological developments. Here we combine atomic force microscopy (AFM) and scanning tunnelling microscopy (STM), supported by first-principles calculations, for the simultaneous imaging and unambiguous identification of atomic species at the (101) anatase surface. We demonstrate that dynamic AFM-STM operation allows atomic resolution imaging within the material's band gap. Based on key distinguishing features extracted from calculations and experiments, we identify candidates for the most common surface defects. Our results pave the way for the understanding of surface processes, like adsorption of metal dopants and photoactive molecules, that are fundamental for the catalytic and photovoltaic applications of anatase, and demonstrate the potential of dynamic AFM-STM for the characterization of wide band gap materials.

No MeSH data available.


Related in: MedlinePlus