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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

Simultaneous AFM and averaged tunnelling current images of the TiO2(101) anatase surface.(a) AFM topographic image representing the general morphology of the surface over a (160 × 160) nm2 area. Acquisition parameters (see Methods) are: fo=170,999 Hz; Δf=−6.5 Hz; A=261.3 Å; K=33.4 N m−1; CPD=VBias=−243 mV. (b,c) Simultaneous topographic AFM (Z(Δf)) and averaged tunnelling current (<It>) data showing a characteristic atomic pattern that displays well-defined ovals along the rows of protrusions appearing in the Z(Δf) image. Acquisition parameters are: fo=153,031 Hz; Δf=−47.4 Hz; A=107.1 Å; K=23.9 N m−1; CPD=VBias=+800 mV. (d,e) Simultaneous Z(Δf) and <It> images obtained with a different tip termination, and corresponding to a less frequent atomic pattern characterized by featureless rows of protrusions in the Z(Δf) image. Acquisition parameters are: fo=158,957 Hz; Δf=−6.0 Hz; A=143.2 Å; K=26.8 N m−1; CPD=VBias=+510 mV. For both sets, image dimensions are (5 × 3) nm2. The parallelogram marks the same surface area in b,c and d,e, respectively. The crystallographic directions of the surface are indicated in a and d. All the experimental images in this work display identical orientation with respect to these directions. (f) Ball-and-stick model of the TiO2(101) anatase surface, which terminates in rows of twofold coordinated oxygen atoms (O2c), followed by a bilayer of threefold coordinated oxygen (O3c, second atomic layer) and fivefold coordinated titanium (Ti5c, third atomic layer) atoms, and a deeper second bilayer of O3c and sixfold coordinated titanium (Ti6c) atoms.
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f1: Simultaneous AFM and averaged tunnelling current images of the TiO2(101) anatase surface.(a) AFM topographic image representing the general morphology of the surface over a (160 × 160) nm2 area. Acquisition parameters (see Methods) are: fo=170,999 Hz; Δf=−6.5 Hz; A=261.3 Å; K=33.4 N m−1; CPD=VBias=−243 mV. (b,c) Simultaneous topographic AFM (Z(Δf)) and averaged tunnelling current (<It>) data showing a characteristic atomic pattern that displays well-defined ovals along the rows of protrusions appearing in the Z(Δf) image. Acquisition parameters are: fo=153,031 Hz; Δf=−47.4 Hz; A=107.1 Å; K=23.9 N m−1; CPD=VBias=+800 mV. (d,e) Simultaneous Z(Δf) and <It> images obtained with a different tip termination, and corresponding to a less frequent atomic pattern characterized by featureless rows of protrusions in the Z(Δf) image. Acquisition parameters are: fo=158,957 Hz; Δf=−6.0 Hz; A=143.2 Å; K=26.8 N m−1; CPD=VBias=+510 mV. For both sets, image dimensions are (5 × 3) nm2. The parallelogram marks the same surface area in b,c and d,e, respectively. The crystallographic directions of the surface are indicated in a and d. All the experimental images in this work display identical orientation with respect to these directions. (f) Ball-and-stick model of the TiO2(101) anatase surface, which terminates in rows of twofold coordinated oxygen atoms (O2c), followed by a bilayer of threefold coordinated oxygen (O3c, second atomic layer) and fivefold coordinated titanium (Ti5c, third atomic layer) atoms, and a deeper second bilayer of O3c and sixfold coordinated titanium (Ti6c) atoms.

Mentions: Figure 1a shows the general morphology of the TiO2(101) anatase surface measured with AFM. Characteristic triangular and truncated trapezoidal terraces and islands are clearly observed, in good agreement with previous STM results14. Typical atomic scale AFM and averaged tunnelling current31 (<It>) images are characterized by rows of signal maxima running along the [010] crystallographic direction (Fig. 1b–e). We have identified two typical atomic patterns in AFM images: pattern A (Fig. 1b) displaying clear ovals along the rows of protrusions; and pattern B (Fig. 1d) showing almost featureless bright rows, and accounting for ∼10% incidence. The variability for the <It> images is wider (see also Supplementary Figs 1–3). Apart from the inherent dependence on the tip–surface separation and bias voltage, the latter points towards a marked dependence of the <It> signal on the nature of the probe termination. The comparison of simultaneous AFM and <It> images reveals that the bright features corresponding to high current signal are mainly located in between the rows of protrusions detected by AFM.


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)

Simultaneous AFM and averaged tunnelling current images of the TiO2(101) anatase surface.(a) AFM topographic image representing the general morphology of the surface over a (160 × 160) nm2 area. Acquisition parameters (see Methods) are: fo=170,999 Hz; Δf=−6.5 Hz; A=261.3 Å; K=33.4 N m−1; CPD=VBias=−243 mV. (b,c) Simultaneous topographic AFM (Z(Δf)) and averaged tunnelling current (<It>) data showing a characteristic atomic pattern that displays well-defined ovals along the rows of protrusions appearing in the Z(Δf) image. Acquisition parameters are: fo=153,031 Hz; Δf=−47.4 Hz; A=107.1 Å; K=23.9 N m−1; CPD=VBias=+800 mV. (d,e) Simultaneous Z(Δf) and <It> images obtained with a different tip termination, and corresponding to a less frequent atomic pattern characterized by featureless rows of protrusions in the Z(Δf) image. Acquisition parameters are: fo=158,957 Hz; Δf=−6.0 Hz; A=143.2 Å; K=26.8 N m−1; CPD=VBias=+510 mV. For both sets, image dimensions are (5 × 3) nm2. The parallelogram marks the same surface area in b,c and d,e, respectively. The crystallographic directions of the surface are indicated in a and d. All the experimental images in this work display identical orientation with respect to these directions. (f) Ball-and-stick model of the TiO2(101) anatase surface, which terminates in rows of twofold coordinated oxygen atoms (O2c), followed by a bilayer of threefold coordinated oxygen (O3c, second atomic layer) and fivefold coordinated titanium (Ti5c, third atomic layer) atoms, and a deeper second bilayer of O3c and sixfold coordinated titanium (Ti6c) atoms.
© Copyright Policy - open-access
Related In: Results  -  Collection

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Show All Figures
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f1: Simultaneous AFM and averaged tunnelling current images of the TiO2(101) anatase surface.(a) AFM topographic image representing the general morphology of the surface over a (160 × 160) nm2 area. Acquisition parameters (see Methods) are: fo=170,999 Hz; Δf=−6.5 Hz; A=261.3 Å; K=33.4 N m−1; CPD=VBias=−243 mV. (b,c) Simultaneous topographic AFM (Z(Δf)) and averaged tunnelling current (<It>) data showing a characteristic atomic pattern that displays well-defined ovals along the rows of protrusions appearing in the Z(Δf) image. Acquisition parameters are: fo=153,031 Hz; Δf=−47.4 Hz; A=107.1 Å; K=23.9 N m−1; CPD=VBias=+800 mV. (d,e) Simultaneous Z(Δf) and <It> images obtained with a different tip termination, and corresponding to a less frequent atomic pattern characterized by featureless rows of protrusions in the Z(Δf) image. Acquisition parameters are: fo=158,957 Hz; Δf=−6.0 Hz; A=143.2 Å; K=26.8 N m−1; CPD=VBias=+510 mV. For both sets, image dimensions are (5 × 3) nm2. The parallelogram marks the same surface area in b,c and d,e, respectively. The crystallographic directions of the surface are indicated in a and d. All the experimental images in this work display identical orientation with respect to these directions. (f) Ball-and-stick model of the TiO2(101) anatase surface, which terminates in rows of twofold coordinated oxygen atoms (O2c), followed by a bilayer of threefold coordinated oxygen (O3c, second atomic layer) and fivefold coordinated titanium (Ti5c, third atomic layer) atoms, and a deeper second bilayer of O3c and sixfold coordinated titanium (Ti6c) atoms.
Mentions: Figure 1a shows the general morphology of the TiO2(101) anatase surface measured with AFM. Characteristic triangular and truncated trapezoidal terraces and islands are clearly observed, in good agreement with previous STM results14. Typical atomic scale AFM and averaged tunnelling current31 (<It>) images are characterized by rows of signal maxima running along the [010] crystallographic direction (Fig. 1b–e). We have identified two typical atomic patterns in AFM images: pattern A (Fig. 1b) displaying clear ovals along the rows of protrusions; and pattern B (Fig. 1d) showing almost featureless bright rows, and accounting for ∼10% incidence. The variability for the <It> images is wider (see also Supplementary Figs 1–3). Apart from the inherent dependence on the tip–surface separation and bias voltage, the latter points towards a marked dependence of the <It> signal on the nature of the probe termination. The comparison of simultaneous AFM and <It> images reveals that the bright features corresponding to high current signal are mainly located in between the rows of protrusions detected by AFM.

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