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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.


Atomic relaxations at the forefront of the tip model on interaction with the surface.Computational snapshots illustrating the evolution in optimal atomistic arrangement of tip and surface atoms as the tip model is approached towards: (a) an O2c atomic site; (b) an Ti5c atomic site, and (c) a hydroxyl defect site of the TiO2(101) anatase surface. Tip height (d) labels below the images are directly related to the tip–surface distance in computed force spectroscopy graphs shown in Fig. 3a,c. At d=3 Å, the relevant hydrogen bond interaction is highlighted with a dashed line. At d=2 Å, the images where the only system change is a small downwards shift of surface atoms due to tip–surface repulsion are omitted.
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f4: Atomic relaxations at the forefront of the tip model on interaction with the surface.Computational snapshots illustrating the evolution in optimal atomistic arrangement of tip and surface atoms as the tip model is approached towards: (a) an O2c atomic site; (b) an Ti5c atomic site, and (c) a hydroxyl defect site of the TiO2(101) anatase surface. Tip height (d) labels below the images are directly related to the tip–surface distance in computed force spectroscopy graphs shown in Fig. 3a,c. At d=3 Å, the relevant hydrogen bond interaction is highlighted with a dashed line. At d=2 Å, the images where the only system change is a small downwards shift of surface atoms due to tip–surface repulsion are omitted.

Mentions: The analysis of the calculated forces corroborates the experimental observations and confirms that, at the onset of the tip–surface interatomic forces, AFM should image the water molecule and the O2c atoms as protrusions. Atomic relaxations of the tip model at the O2c site demonstrate that hydrogen bond formation between the hydroxyl group at the AFM probe and the surface oxygen atom dominates the interaction (Fig. 4a). In the same tip–surface separation regime, such atomic relaxations were also observed with tips probing the close-by Ti6c and Ti5c sites: the hydroxyl group at the probe re-orients towards the nearest O2c site and a hydrogen bond is formed (Fig. 4b). Such tip relaxation effects explain the most common topographic AFM images (Fig. 1b) that exhibit rows of protruding ovals slightly elongated along the crystallographic direction. Placing the tip model into a lower height regime reveals a double minima feature in the force curve over the Ti5c site. This feature is due to the deflection of the tip hydrogen (H) atom at the first force minimum that further allows the exposed oxygen of the hydroxyl to engage in bonding with the sampled Ti5c atom at the second minimum (Fig. 4b, d=2 Å). Traces of double force minima at Ti5c sites can also be observed experimentally for some probes, as displayed in Fig. 3a. The O3c surface atoms are laterally positioned furthest away from the O2c sites, so the force curves computed over them are free of the hydrogen bond interaction with the O2c atoms. The small forces predicted over the shallower O3c atoms should make them almost undetectable by the AFM. Force spectroscopy at most atomic sites of the clean anatase surface—excluding the O3c atoms—results in the tip hydroxyl group reorienting towards the nearest O2c site, which leads to repeated sampling of the same chemical interaction and an extended, oval-shape maxima in the experimental AFM 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)

Atomic relaxations at the forefront of the tip model on interaction with the surface.Computational snapshots illustrating the evolution in optimal atomistic arrangement of tip and surface atoms as the tip model is approached towards: (a) an O2c atomic site; (b) an Ti5c atomic site, and (c) a hydroxyl defect site of the TiO2(101) anatase surface. Tip height (d) labels below the images are directly related to the tip–surface distance in computed force spectroscopy graphs shown in Fig. 3a,c. At d=3 Å, the relevant hydrogen bond interaction is highlighted with a dashed line. At d=2 Å, the images where the only system change is a small downwards shift of surface atoms due to tip–surface repulsion are omitted.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Atomic relaxations at the forefront of the tip model on interaction with the surface.Computational snapshots illustrating the evolution in optimal atomistic arrangement of tip and surface atoms as the tip model is approached towards: (a) an O2c atomic site; (b) an Ti5c atomic site, and (c) a hydroxyl defect site of the TiO2(101) anatase surface. Tip height (d) labels below the images are directly related to the tip–surface distance in computed force spectroscopy graphs shown in Fig. 3a,c. At d=3 Å, the relevant hydrogen bond interaction is highlighted with a dashed line. At d=2 Å, the images where the only system change is a small downwards shift of surface atoms due to tip–surface repulsion are omitted.
Mentions: The analysis of the calculated forces corroborates the experimental observations and confirms that, at the onset of the tip–surface interatomic forces, AFM should image the water molecule and the O2c atoms as protrusions. Atomic relaxations of the tip model at the O2c site demonstrate that hydrogen bond formation between the hydroxyl group at the AFM probe and the surface oxygen atom dominates the interaction (Fig. 4a). In the same tip–surface separation regime, such atomic relaxations were also observed with tips probing the close-by Ti6c and Ti5c sites: the hydroxyl group at the probe re-orients towards the nearest O2c site and a hydrogen bond is formed (Fig. 4b). Such tip relaxation effects explain the most common topographic AFM images (Fig. 1b) that exhibit rows of protruding ovals slightly elongated along the crystallographic direction. Placing the tip model into a lower height regime reveals a double minima feature in the force curve over the Ti5c site. This feature is due to the deflection of the tip hydrogen (H) atom at the first force minimum that further allows the exposed oxygen of the hydroxyl to engage in bonding with the sampled Ti5c atom at the second minimum (Fig. 4b, d=2 Å). Traces of double force minima at Ti5c sites can also be observed experimentally for some probes, as displayed in Fig. 3a. The O3c surface atoms are laterally positioned furthest away from the O2c sites, so the force curves computed over them are free of the hydrogen bond interaction with the O2c atoms. The small forces predicted over the shallower O3c atoms should make them almost undetectable by the AFM. Force spectroscopy at most atomic sites of the clean anatase surface—excluding the O3c atoms—results in the tip hydroxyl group reorienting towards the nearest O2c site, which leads to repeated sampling of the same chemical interaction and an extended, oval-shape maxima in the experimental AFM 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.