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Identification of different oxygen species in oxide nanostructures with (17)O solid-state NMR spectroscopy.

Wang M, Wu XP, Zheng S, Zhao L, Li L, Shen L, Gao Y, Xue N, Guo X, Huang W, Gan Z, Blanc F, Yu Z, Ke X, Ding W, Gong XQ, Grey CP, Peng L - Sci Adv (2015)

Bottom Line: We show that the (17)O resonances arising from the first to third surface layer oxygen ions, hydroxyl sites, and oxygen species near vacancies can be distinguished from the oxygen ions in the bulk, with higher-frequency (17)O chemical shifts being observed for the lower coordinated surface sites.H2 (17)O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. (17)O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature.These results open up new strategies for characterizing nanostructured oxides and their applications.

View Article: PubMed Central - PubMed

Affiliation: Key Laboratory of Mesoscopic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China.

ABSTRACT
Nanostructured oxides find multiple uses in a diverse range of applications including catalysis, energy storage, and environmental management, their higher surface areas, and, in some cases, electronic properties resulting in different physical properties from their bulk counterparts. Developing structure-property relations for these materials requires a determination of surface and subsurface structure. Although microscopy plays a critical role owing to the fact that the volumes sampled by such techniques may not be representative of the whole sample, complementary characterization methods are urgently required. We develop a simple nuclear magnetic resonance (NMR) strategy to detect the first few layers of a nanomaterial, demonstrating the approach with technologically relevant ceria nanoparticles. We show that the (17)O resonances arising from the first to third surface layer oxygen ions, hydroxyl sites, and oxygen species near vacancies can be distinguished from the oxygen ions in the bulk, with higher-frequency (17)O chemical shifts being observed for the lower coordinated surface sites. H2 (17)O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. (17)O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature. Density functional theory calculations confirm the assignments and reveal a strong dependence of chemical shift on the nature of the surface. These results open up new strategies for characterizing nanostructured oxides and their applications.

No MeSH data available.


Related in: MedlinePlus

Solid-state NMR spectra of ceria in contact with water.(A, bottom to top) 17O MAS NMR spectra of nonenriched ceria nanoparticles adsorbed with nonenriched water; ceria nanoparticles enriched in 17O2 at 573 K and then exposed to air; ceria nanoparticles enriched in 17O2 at 573 K and then adsorbed with nonenriched water (water was added dropwise); the previous sample dried under vacuum at room temperature and at 573 K; and ceria nanoparticles enriched in 17O2 at 573 K. (B, bottom to top) 17O NMR spectra of liquid H217O; 17O MAS NMR spectra of bulk ceria adsorbed with H217O (bulk-H217O); the previous sample dried under vacuum at 373 K; nonenriched ceria nanoparticles adsorbed with H217O by adding water dropwise [nano-H217O(D)]; the previous sample dried under vacuum at 373 K; and nonenriched ceria nanoparticles adsorbed with H217O by adsorbing water through a vacuum line and then dried under vacuum at 373 K [nano-H217O(A)]. The samples of H217O, bulk-H217O, and nano-H217O(D) in (B) were packed into the rotors in air. All the spectra were acquired at 9.4 T. Detailed experimental parameters are summarized in table S10. Asterisks denote spinning sidebands.
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Figure 2: Solid-state NMR spectra of ceria in contact with water.(A, bottom to top) 17O MAS NMR spectra of nonenriched ceria nanoparticles adsorbed with nonenriched water; ceria nanoparticles enriched in 17O2 at 573 K and then exposed to air; ceria nanoparticles enriched in 17O2 at 573 K and then adsorbed with nonenriched water (water was added dropwise); the previous sample dried under vacuum at room temperature and at 573 K; and ceria nanoparticles enriched in 17O2 at 573 K. (B, bottom to top) 17O NMR spectra of liquid H217O; 17O MAS NMR spectra of bulk ceria adsorbed with H217O (bulk-H217O); the previous sample dried under vacuum at 373 K; nonenriched ceria nanoparticles adsorbed with H217O by adding water dropwise [nano-H217O(D)]; the previous sample dried under vacuum at 373 K; and nonenriched ceria nanoparticles adsorbed with H217O by adsorbing water through a vacuum line and then dried under vacuum at 373 K [nano-H217O(A)]. The samples of H217O, bulk-H217O, and nano-H217O(D) in (B) were packed into the rotors in air. All the spectra were acquired at 9.4 T. Detailed experimental parameters are summarized in table S10. Asterisks denote spinning sidebands.

Mentions: To provide additional support for the spectral assignments and to explore the surface structure and chemistry of ceria nanoparticles, natural abundance water was adsorbed on the 17O-enriched ceria samples. The major 17O resonance at 877 ppm from OCe4 (bulk ceria) remains in the NMR spectra of these samples (Fig. 2A) because the oxygen ions in the bulk are not expected to be affected by surface adsorbates. The resonances at about 1040, 920, and 825 ppm, however, disappear and a new broad shoulder at about 850 ppm can be observed. The shift of this resonance remains unchanged in the spectrum acquired at an ultrahigh field of 19.4 T, indicating that the resonance is associated with a small CQ (figs. S7 and S8A and table S4). In addition, another broad peak is observed at a much lower frequency (32 ppm). This peak disappears after the sample is dried under vacuum at room temperature, suggesting that this resonance can be assigned to water adsorbed on the surface of ceria. After thermal treatment of the sample at 573 K, the broad peak at 850 ppm disappears and the peaks at 1040, 920, and 825 ppm can be observed again, and the spectrum resembles that of the original bare ceria nanoparticles; this indicates that the structure of the ceria nanoparticles is recovered at elevated temperature when water is completely removed. The resonance at 850 ppm is ascribed to the oxygen ions in the third layer (these environments resonating at 825 ppm in the dry sample), in agreement with DFT calculations (fig. S8 and table S5). The resonances from the oxygen ions on the first and second layers are not observed when water is adsorbed, presumably because of fast exchange between these oxygen ions with water; this hypothesis will be further explored below.


Identification of different oxygen species in oxide nanostructures with (17)O solid-state NMR spectroscopy.

Wang M, Wu XP, Zheng S, Zhao L, Li L, Shen L, Gao Y, Xue N, Guo X, Huang W, Gan Z, Blanc F, Yu Z, Ke X, Ding W, Gong XQ, Grey CP, Peng L - Sci Adv (2015)

Solid-state NMR spectra of ceria in contact with water.(A, bottom to top) 17O MAS NMR spectra of nonenriched ceria nanoparticles adsorbed with nonenriched water; ceria nanoparticles enriched in 17O2 at 573 K and then exposed to air; ceria nanoparticles enriched in 17O2 at 573 K and then adsorbed with nonenriched water (water was added dropwise); the previous sample dried under vacuum at room temperature and at 573 K; and ceria nanoparticles enriched in 17O2 at 573 K. (B, bottom to top) 17O NMR spectra of liquid H217O; 17O MAS NMR spectra of bulk ceria adsorbed with H217O (bulk-H217O); the previous sample dried under vacuum at 373 K; nonenriched ceria nanoparticles adsorbed with H217O by adding water dropwise [nano-H217O(D)]; the previous sample dried under vacuum at 373 K; and nonenriched ceria nanoparticles adsorbed with H217O by adsorbing water through a vacuum line and then dried under vacuum at 373 K [nano-H217O(A)]. The samples of H217O, bulk-H217O, and nano-H217O(D) in (B) were packed into the rotors in air. All the spectra were acquired at 9.4 T. Detailed experimental parameters are summarized in table S10. Asterisks denote spinning sidebands.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4644084&req=5

Figure 2: Solid-state NMR spectra of ceria in contact with water.(A, bottom to top) 17O MAS NMR spectra of nonenriched ceria nanoparticles adsorbed with nonenriched water; ceria nanoparticles enriched in 17O2 at 573 K and then exposed to air; ceria nanoparticles enriched in 17O2 at 573 K and then adsorbed with nonenriched water (water was added dropwise); the previous sample dried under vacuum at room temperature and at 573 K; and ceria nanoparticles enriched in 17O2 at 573 K. (B, bottom to top) 17O NMR spectra of liquid H217O; 17O MAS NMR spectra of bulk ceria adsorbed with H217O (bulk-H217O); the previous sample dried under vacuum at 373 K; nonenriched ceria nanoparticles adsorbed with H217O by adding water dropwise [nano-H217O(D)]; the previous sample dried under vacuum at 373 K; and nonenriched ceria nanoparticles adsorbed with H217O by adsorbing water through a vacuum line and then dried under vacuum at 373 K [nano-H217O(A)]. The samples of H217O, bulk-H217O, and nano-H217O(D) in (B) were packed into the rotors in air. All the spectra were acquired at 9.4 T. Detailed experimental parameters are summarized in table S10. Asterisks denote spinning sidebands.
Mentions: To provide additional support for the spectral assignments and to explore the surface structure and chemistry of ceria nanoparticles, natural abundance water was adsorbed on the 17O-enriched ceria samples. The major 17O resonance at 877 ppm from OCe4 (bulk ceria) remains in the NMR spectra of these samples (Fig. 2A) because the oxygen ions in the bulk are not expected to be affected by surface adsorbates. The resonances at about 1040, 920, and 825 ppm, however, disappear and a new broad shoulder at about 850 ppm can be observed. The shift of this resonance remains unchanged in the spectrum acquired at an ultrahigh field of 19.4 T, indicating that the resonance is associated with a small CQ (figs. S7 and S8A and table S4). In addition, another broad peak is observed at a much lower frequency (32 ppm). This peak disappears after the sample is dried under vacuum at room temperature, suggesting that this resonance can be assigned to water adsorbed on the surface of ceria. After thermal treatment of the sample at 573 K, the broad peak at 850 ppm disappears and the peaks at 1040, 920, and 825 ppm can be observed again, and the spectrum resembles that of the original bare ceria nanoparticles; this indicates that the structure of the ceria nanoparticles is recovered at elevated temperature when water is completely removed. The resonance at 850 ppm is ascribed to the oxygen ions in the third layer (these environments resonating at 825 ppm in the dry sample), in agreement with DFT calculations (fig. S8 and table S5). The resonances from the oxygen ions on the first and second layers are not observed when water is adsorbed, presumably because of fast exchange between these oxygen ions with water; this hypothesis will be further explored below.

Bottom Line: We show that the (17)O resonances arising from the first to third surface layer oxygen ions, hydroxyl sites, and oxygen species near vacancies can be distinguished from the oxygen ions in the bulk, with higher-frequency (17)O chemical shifts being observed for the lower coordinated surface sites.H2 (17)O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. (17)O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature.These results open up new strategies for characterizing nanostructured oxides and their applications.

View Article: PubMed Central - PubMed

Affiliation: Key Laboratory of Mesoscopic Chemistry of Ministry of Education, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China.

ABSTRACT
Nanostructured oxides find multiple uses in a diverse range of applications including catalysis, energy storage, and environmental management, their higher surface areas, and, in some cases, electronic properties resulting in different physical properties from their bulk counterparts. Developing structure-property relations for these materials requires a determination of surface and subsurface structure. Although microscopy plays a critical role owing to the fact that the volumes sampled by such techniques may not be representative of the whole sample, complementary characterization methods are urgently required. We develop a simple nuclear magnetic resonance (NMR) strategy to detect the first few layers of a nanomaterial, demonstrating the approach with technologically relevant ceria nanoparticles. We show that the (17)O resonances arising from the first to third surface layer oxygen ions, hydroxyl sites, and oxygen species near vacancies can be distinguished from the oxygen ions in the bulk, with higher-frequency (17)O chemical shifts being observed for the lower coordinated surface sites. H2 (17)O can be used to selectively enrich surface sites, allowing only these particular active sites to be monitored in a chemical process. (17)O NMR spectra of thermally treated nanosized ceria clearly show how different oxygen species interconvert at elevated temperature. Density functional theory calculations confirm the assignments and reveal a strong dependence of chemical shift on the nature of the surface. These results open up new strategies for characterizing nanostructured oxides and their applications.

No MeSH data available.


Related in: MedlinePlus