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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

Individual water molecules as atomic markers.(a,b) Simultaneous topographic AFM (Z(Δf)) and averaged tunnelling current (<It>) images showing four individual water molecules adsorbed on the TiO2(101) anatase surface. Image dimensions are (4.5 × 4.5) nm2. Simultaneous Z(Δf) (c) and <It> (d) signals ascribed to a single water molecule imaged with a different—more symmetric—tip termination. A top view of the outer atomic layers of the TiO2(101) anatase surface has been superimposed to the images (see text for details). The squares mark the Ti5c atom at which the water molecule binds to the surface17. The circles highlight the O2c atoms that sustain two weak hydrogen bonds with the water molecule17. (e) Sets of simultaneous Z(Δf) and <It> bias-dependent images obtained over the water molecule displayed in c and d with identical tip termination and approximately keeping the same tip–surface separation (see Methods). The contrast of the filled state images (negative sample bias voltage) has been inverted (−<It>) for a better comparison with the empty state data. These images without the atomic model of the anatase (101) surface superimposed are displayed in Supplementary Fig. 4. c and d are a magnification of the images labelled as −400 mV in e. Image dimensions are (2 × 2) nm2. Acquisition parameters are: fo=159,989 Hz, Δf=−6.6 Hz, A=118.0 Å, K=27.3 N m−1, CPD=VBias=+400 mV, for a and b; and fo=159,989 Hz, A=113.2 Å, K=27.3 N m−1, CPD=−180 mV for c–e. The Δf set point and the VBias value are listed under each set of images in e.
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f2: Individual water molecules as atomic markers.(a,b) Simultaneous topographic AFM (Z(Δf)) and averaged tunnelling current (<It>) images showing four individual water molecules adsorbed on the TiO2(101) anatase surface. Image dimensions are (4.5 × 4.5) nm2. Simultaneous Z(Δf) (c) and <It> (d) signals ascribed to a single water molecule imaged with a different—more symmetric—tip termination. A top view of the outer atomic layers of the TiO2(101) anatase surface has been superimposed to the images (see text for details). The squares mark the Ti5c atom at which the water molecule binds to the surface17. The circles highlight the O2c atoms that sustain two weak hydrogen bonds with the water molecule17. (e) Sets of simultaneous Z(Δf) and <It> bias-dependent images obtained over the water molecule displayed in c and d with identical tip termination and approximately keeping the same tip–surface separation (see Methods). The contrast of the filled state images (negative sample bias voltage) has been inverted (−<It>) for a better comparison with the empty state data. These images without the atomic model of the anatase (101) surface superimposed are displayed in Supplementary Fig. 4. c and d are a magnification of the images labelled as −400 mV in e. Image dimensions are (2 × 2) nm2. Acquisition parameters are: fo=159,989 Hz, Δf=−6.6 Hz, A=118.0 Å, K=27.3 N m−1, CPD=VBias=+400 mV, for a and b; and fo=159,989 Hz, A=113.2 Å, K=27.3 N m−1, CPD=−180 mV for c–e. The Δf set point and the VBias value are listed under each set of images in e.

Mentions: In early STM studies on anatase surfaces, the assignment of the imaged atomic species was normally presumed following the assumption that at sample bias voltages close to the conduction band, the Ti atoms should contribute the most to the tunnelling current, by analogy with the case of the TiO2(110) rutile surface32. However, the contribution of the surface atoms to the topographic STM images is subject to the current set point and the bias voltage17. In this work, to experimentally verify the contribution of the surface atomic species to our AFM and <It> images, we use individual water molecules—intentionally deposited on the surface—as atomic markers (Fig. 2). Previous theoretical works have shown that the oxygen atom of a water molecule adsorbed on anatase (101) strongly binds to one of the Ti5c atoms17. This bond produces a redistribution of the local density of the states around the targeted Ti5c atom that makes it vanish from an STM image17. The water molecule additionally sustains two weak hydrogen bonds with O2c atoms at the closest oxygen atomic row17. These bonding features are clearly identified in our atomic resolution images.


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)

Individual water molecules as atomic markers.(a,b) Simultaneous topographic AFM (Z(Δf)) and averaged tunnelling current (<It>) images showing four individual water molecules adsorbed on the TiO2(101) anatase surface. Image dimensions are (4.5 × 4.5) nm2. Simultaneous Z(Δf) (c) and <It> (d) signals ascribed to a single water molecule imaged with a different—more symmetric—tip termination. A top view of the outer atomic layers of the TiO2(101) anatase surface has been superimposed to the images (see text for details). The squares mark the Ti5c atom at which the water molecule binds to the surface17. The circles highlight the O2c atoms that sustain two weak hydrogen bonds with the water molecule17. (e) Sets of simultaneous Z(Δf) and <It> bias-dependent images obtained over the water molecule displayed in c and d with identical tip termination and approximately keeping the same tip–surface separation (see Methods). The contrast of the filled state images (negative sample bias voltage) has been inverted (−<It>) for a better comparison with the empty state data. These images without the atomic model of the anatase (101) surface superimposed are displayed in Supplementary Fig. 4. c and d are a magnification of the images labelled as −400 mV in e. Image dimensions are (2 × 2) nm2. Acquisition parameters are: fo=159,989 Hz, Δf=−6.6 Hz, A=118.0 Å, K=27.3 N m−1, CPD=VBias=+400 mV, for a and b; and fo=159,989 Hz, A=113.2 Å, K=27.3 N m−1, CPD=−180 mV for c–e. The Δf set point and the VBias value are listed under each set of images in e.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
getmorefigures.php?uid=PMC4491188&req=5

f2: Individual water molecules as atomic markers.(a,b) Simultaneous topographic AFM (Z(Δf)) and averaged tunnelling current (<It>) images showing four individual water molecules adsorbed on the TiO2(101) anatase surface. Image dimensions are (4.5 × 4.5) nm2. Simultaneous Z(Δf) (c) and <It> (d) signals ascribed to a single water molecule imaged with a different—more symmetric—tip termination. A top view of the outer atomic layers of the TiO2(101) anatase surface has been superimposed to the images (see text for details). The squares mark the Ti5c atom at which the water molecule binds to the surface17. The circles highlight the O2c atoms that sustain two weak hydrogen bonds with the water molecule17. (e) Sets of simultaneous Z(Δf) and <It> bias-dependent images obtained over the water molecule displayed in c and d with identical tip termination and approximately keeping the same tip–surface separation (see Methods). The contrast of the filled state images (negative sample bias voltage) has been inverted (−<It>) for a better comparison with the empty state data. These images without the atomic model of the anatase (101) surface superimposed are displayed in Supplementary Fig. 4. c and d are a magnification of the images labelled as −400 mV in e. Image dimensions are (2 × 2) nm2. Acquisition parameters are: fo=159,989 Hz, Δf=−6.6 Hz, A=118.0 Å, K=27.3 N m−1, CPD=VBias=+400 mV, for a and b; and fo=159,989 Hz, A=113.2 Å, K=27.3 N m−1, CPD=−180 mV for c–e. The Δf set point and the VBias value are listed under each set of images in e.
Mentions: In early STM studies on anatase surfaces, the assignment of the imaged atomic species was normally presumed following the assumption that at sample bias voltages close to the conduction band, the Ti atoms should contribute the most to the tunnelling current, by analogy with the case of the TiO2(110) rutile surface32. However, the contribution of the surface atoms to the topographic STM images is subject to the current set point and the bias voltage17. In this work, to experimentally verify the contribution of the surface atomic species to our AFM and <It> images, we use individual water molecules—intentionally deposited on the surface—as atomic markers (Fig. 2). Previous theoretical works have shown that the oxygen atom of a water molecule adsorbed on anatase (101) strongly binds to one of the Ti5c atoms17. This bond produces a redistribution of the local density of the states around the targeted Ti5c atom that makes it vanish from an STM image17. The water molecule additionally sustains two weak hydrogen bonds with O2c atoms at the closest oxygen atomic row17. These bonding features are clearly identified in our atomic resolution images.

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