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Engineering surface atomic structure of single-crystal cobalt (II) oxide nanorods for superior electrocatalysis

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ABSTRACT

Engineering the surface structure at the atomic level can be used to precisely and effectively manipulate the reactivity and durability of catalysts. Here we report tuning of the atomic structure of one-dimensional single-crystal cobalt (II) oxide (CoO) nanorods by creating oxygen vacancies on pyramidal nanofacets. These CoO nanorods exhibit superior catalytic activity and durability towards oxygen reduction/evolution reactions. The combined experimental studies, microscopic and spectroscopic characterization, and density functional theory calculations reveal that the origins of the electrochemical activity of single-crystal CoO nanorods are in the oxygen vacancies that can be readily created on the oxygen-terminated {111} nanofacets, which favourably affect the electronic structure of CoO, assuring a rapid charge transfer and optimal adsorption energies for intermediates of oxygen reduction/evolution reactions. These results show that the surface atomic structure engineering is important for the fabrication of efficient and durable electrocatalysts.

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Structural characterization of SC CoO NRs.(a) Top-view SEM image of SC CoO NRs fabricated directly on CFP. Scale bar, 10 μm. The inset in a shows morphology of SC CoO NRs. Scale bar, 1 μm. (b) High magnification TEM image of an individual SC CoO NR with saw-like edges. Scale bar, 20 nm. (c) High-resolution HAADF-STEM image taken from the outermost surface of a single SC CoO NR revealing the exposed {111} nanofacets (indicated by orange and green arrows), with inset showing the corresponding selected area electron diffraction pattern taken from [110] zone axis. Scale bar, 2 nm. (d) Atomic model of a nanopyramid enclosed with {100} and {111} facets, and (e) the projection of this pyramidal structure along [110] zone axis. (f,g) Experimental and simulated HADDF-STEM images of the pyramidal structure, respectively. Scale bar in f, 1 nm. Note that inelastic and neutron scatterings were not considered in the simulation, which contribute to the background in h. (h,i) The intensity profiles taken from orange and grey lines in f and g, respectively.
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f2: Structural characterization of SC CoO NRs.(a) Top-view SEM image of SC CoO NRs fabricated directly on CFP. Scale bar, 10 μm. The inset in a shows morphology of SC CoO NRs. Scale bar, 1 μm. (b) High magnification TEM image of an individual SC CoO NR with saw-like edges. Scale bar, 20 nm. (c) High-resolution HAADF-STEM image taken from the outermost surface of a single SC CoO NR revealing the exposed {111} nanofacets (indicated by orange and green arrows), with inset showing the corresponding selected area electron diffraction pattern taken from [110] zone axis. Scale bar, 2 nm. (d) Atomic model of a nanopyramid enclosed with {100} and {111} facets, and (e) the projection of this pyramidal structure along [110] zone axis. (f,g) Experimental and simulated HADDF-STEM images of the pyramidal structure, respectively. Scale bar in f, 1 nm. Note that inelastic and neutron scatterings were not considered in the simulation, which contribute to the background in h. (h,i) The intensity profiles taken from orange and grey lines in f and g, respectively.

Mentions: The microstructure of as-synthesized CoO NRs was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As displayed in Fig. 2a, the entire surface of CFP is uniformly covered with CoO NRs, which are SCs (Fig. 2c, inset). Interestingly, after cation exchange reaction, numerous nanopores with sizes of 5–20 nm are visible on the surface and across NRs (Supplementary Fig. 5). Surprisingly, the surface of CoO NRs becomes rather rough as evidenced by tooth-like growths with sizes of about 5 nm (Fig. 2b). Atomic level high-angle annular dark field-scanning TEM (HADDF-STEM) image shows that these growths are sharply terminated with {111} nanofacets (Fig. 2c). Notably, the gradual contrast variation in these single tooth-like growths suggests a progressive variation in their thickness (Fig. 2c). Simulation of experimental image was performed to accurately determine the three-dimensional atomic arrangement in the aforementioned growths. A speculated nanopyramidal structure with exposed {100} and {111} facets was constructed as shown in Fig. 2d,e. Figure 2f,g indicates a good agreement between the experimental and simulated images. Moreover, the intensity profile along the terminated {111} facet in the experimental image (Fig. 2h) closely resembles that in the simulated one (Fig. 2i). A good match between experimental and simulated HADDF-STEM images clearly demonstrates that the surface of SC CoO NRs is surrounded by nanopyramids and preferentially exposed {111} facets. The surface area of exposed {111} facets is estimated to be 46% of the total surface area of SC CoO NRs (Supplementary Fig. 6 and Supplementary Note 1). Notably, for CoO the surface energy of {111} is much higher than that of other low-indexed facets44. Such high percentage of {111} facets without foreign stabilizer is rather difficult to achieve via thermodynamically controlled synthesis44. However, in our kinetics-governed cation exchange strategy, facets with high surface energy and defects (discussed later) are forced to be exposed to facilitate the ion exchange process45, assuring the formation of a large amount of clean and defect-rich {111} facets on the surface of SC CoO NRs, which is certainly highly preferable for catalysis. Furthermore, it should be noted that {111} facets of the bulk CoO are polar, either terminated by oxygen (O) or Co-atomic layer46. A detailed X-ray photoelectron spectroscopy (XPS) analysis indicates that the exposed {111} facets on CoO NRs should be O-terminated (Supplementary Fig. 7, Supplementary Table 1 and Supplementary Note 2).


Engineering surface atomic structure of single-crystal cobalt (II) oxide nanorods for superior electrocatalysis
Structural characterization of SC CoO NRs.(a) Top-view SEM image of SC CoO NRs fabricated directly on CFP. Scale bar, 10 μm. The inset in a shows morphology of SC CoO NRs. Scale bar, 1 μm. (b) High magnification TEM image of an individual SC CoO NR with saw-like edges. Scale bar, 20 nm. (c) High-resolution HAADF-STEM image taken from the outermost surface of a single SC CoO NR revealing the exposed {111} nanofacets (indicated by orange and green arrows), with inset showing the corresponding selected area electron diffraction pattern taken from [110] zone axis. Scale bar, 2 nm. (d) Atomic model of a nanopyramid enclosed with {100} and {111} facets, and (e) the projection of this pyramidal structure along [110] zone axis. (f,g) Experimental and simulated HADDF-STEM images of the pyramidal structure, respectively. Scale bar in f, 1 nm. Note that inelastic and neutron scatterings were not considered in the simulation, which contribute to the background in h. (h,i) The intensity profiles taken from orange and grey lines in f and g, respectively.
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Related In: Results  -  Collection

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f2: Structural characterization of SC CoO NRs.(a) Top-view SEM image of SC CoO NRs fabricated directly on CFP. Scale bar, 10 μm. The inset in a shows morphology of SC CoO NRs. Scale bar, 1 μm. (b) High magnification TEM image of an individual SC CoO NR with saw-like edges. Scale bar, 20 nm. (c) High-resolution HAADF-STEM image taken from the outermost surface of a single SC CoO NR revealing the exposed {111} nanofacets (indicated by orange and green arrows), with inset showing the corresponding selected area electron diffraction pattern taken from [110] zone axis. Scale bar, 2 nm. (d) Atomic model of a nanopyramid enclosed with {100} and {111} facets, and (e) the projection of this pyramidal structure along [110] zone axis. (f,g) Experimental and simulated HADDF-STEM images of the pyramidal structure, respectively. Scale bar in f, 1 nm. Note that inelastic and neutron scatterings were not considered in the simulation, which contribute to the background in h. (h,i) The intensity profiles taken from orange and grey lines in f and g, respectively.
Mentions: The microstructure of as-synthesized CoO NRs was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As displayed in Fig. 2a, the entire surface of CFP is uniformly covered with CoO NRs, which are SCs (Fig. 2c, inset). Interestingly, after cation exchange reaction, numerous nanopores with sizes of 5–20 nm are visible on the surface and across NRs (Supplementary Fig. 5). Surprisingly, the surface of CoO NRs becomes rather rough as evidenced by tooth-like growths with sizes of about 5 nm (Fig. 2b). Atomic level high-angle annular dark field-scanning TEM (HADDF-STEM) image shows that these growths are sharply terminated with {111} nanofacets (Fig. 2c). Notably, the gradual contrast variation in these single tooth-like growths suggests a progressive variation in their thickness (Fig. 2c). Simulation of experimental image was performed to accurately determine the three-dimensional atomic arrangement in the aforementioned growths. A speculated nanopyramidal structure with exposed {100} and {111} facets was constructed as shown in Fig. 2d,e. Figure 2f,g indicates a good agreement between the experimental and simulated images. Moreover, the intensity profile along the terminated {111} facet in the experimental image (Fig. 2h) closely resembles that in the simulated one (Fig. 2i). A good match between experimental and simulated HADDF-STEM images clearly demonstrates that the surface of SC CoO NRs is surrounded by nanopyramids and preferentially exposed {111} facets. The surface area of exposed {111} facets is estimated to be 46% of the total surface area of SC CoO NRs (Supplementary Fig. 6 and Supplementary Note 1). Notably, for CoO the surface energy of {111} is much higher than that of other low-indexed facets44. Such high percentage of {111} facets without foreign stabilizer is rather difficult to achieve via thermodynamically controlled synthesis44. However, in our kinetics-governed cation exchange strategy, facets with high surface energy and defects (discussed later) are forced to be exposed to facilitate the ion exchange process45, assuring the formation of a large amount of clean and defect-rich {111} facets on the surface of SC CoO NRs, which is certainly highly preferable for catalysis. Furthermore, it should be noted that {111} facets of the bulk CoO are polar, either terminated by oxygen (O) or Co-atomic layer46. A detailed X-ray photoelectron spectroscopy (XPS) analysis indicates that the exposed {111} facets on CoO NRs should be O-terminated (Supplementary Fig. 7, Supplementary Table 1 and Supplementary Note 2).

View Article: PubMed Central - PubMed

ABSTRACT

Engineering the surface structure at the atomic level can be used to precisely and effectively manipulate the reactivity and durability of catalysts. Here we report tuning of the atomic structure of one-dimensional single-crystal cobalt (II) oxide (CoO) nanorods by creating oxygen vacancies on pyramidal nanofacets. These CoO nanorods exhibit superior catalytic activity and durability towards oxygen reduction/evolution reactions. The combined experimental studies, microscopic and spectroscopic characterization, and density functional theory calculations reveal that the origins of the electrochemical activity of single-crystal CoO nanorods are in the oxygen vacancies that can be readily created on the oxygen-terminated {111} nanofacets, which favourably affect the electronic structure of CoO, assuring a rapid charge transfer and optimal adsorption energies for intermediates of oxygen reduction/evolution reactions. These results show that the surface atomic structure engineering is important for the fabrication of efficient and durable electrocatalysts.

No MeSH data available.


Related in: MedlinePlus