Limits...
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

Double-resonance solid-state NMR data of ceria nanoparticles.(A, top to bottom) 17O MAS NMR spectra of ceria nanoparticles adsorbed with 17O water followed by thermal treatment under vacuum at 373 K at 14.1 and 9.4 T; difference spectrum in 17O-1H REDOR experiments; and 1H-17O CP MAS NMR spectrum with a contact time of 100 μs. (B) The peak intensity in CP MAS NMR experiment as a function of contact time. Spectra were obtained at both 9.4 and 14.1 T under MAS rates of 13 to 14 kHz. One thousand to 40,000 scans were averaged, and recycle delays from 0.2 to 1 s were used. Asterisks denote spinning sidebands.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC4644084&req=5

Figure 3: Double-resonance solid-state NMR data of ceria nanoparticles.(A, top to bottom) 17O MAS NMR spectra of ceria nanoparticles adsorbed with 17O water followed by thermal treatment under vacuum at 373 K at 14.1 and 9.4 T; difference spectrum in 17O-1H REDOR experiments; and 1H-17O CP MAS NMR spectrum with a contact time of 100 μs. (B) The peak intensity in CP MAS NMR experiment as a function of contact time. Spectra were obtained at both 9.4 and 14.1 T under MAS rates of 13 to 14 kHz. One thousand to 40,000 scans were averaged, and recycle delays from 0.2 to 1 s were used. Asterisks denote spinning sidebands.

Mentions: Interactions between ceria nanoparticles and water were also investigated in a reverse way, through the adsorption of 17O-enriched water on nonenriched ceria nanoparticles (preheated at 573 K). On adding H217O dropwise to ceria nanoparticles at room temperature, weak peaks at about 1040, 877, and 270 ppm appeared (Fig. 2B) in addition to the broad peak at about 32 ppm from adsorbed water. These peaks dominated the spectra after the sample was dried under vacuum at 373 K. Similar spectra were obtained by introducing H217O to nonenriched ceria nanoparticles calcined at 573 K on a vacuum line (Fig. 2B). The broad resonance at 270 ppm can be tentatively assigned to the hydroxyl groups (Ce4+-17OH) observed previously by Fourier transform infrared (20) on the surface of ceria nanoparticles on the basis of the low chemical shift. 17O-1H double-resonance NMR techniques [in this case, cross polarization (CP) (21) and rotational echo double resonance (REDOR) (22)] were used to confirm this assignment because they can be used to select the 17O resonances of oxygen ions close to protons: they measure the heteronuclear dipolar coupling (that is, between 17O and 1H), a function of distance between 17O and 1H. As seen from Fig. 3A, both the REDOR difference spectrum and CP spectra only show one signal at 270 ppm, confirming that this resonance comes from oxygen ions in close proximity to proton. The NMR parameters of this species, including chemical shift (δiso) and quadrupolar product [PQ = CQ(1 + ηQ2/3)1/2], were extracted by calculating the frequency change of the center of gravity of the signal at different external fields (300 ppm at 14.1 T versus 270 ppm at 9.4 T). The obtained values (δiso = 325 ppm, PQ = 5.1 MHz) are also supported by the DFT calculation results (fig. S8 and table S5). It is clear that the surface hydroxyl groups are associated with much larger CQ than the less coordinated surface oxygen species, and this is in agreement with the nutation curve (fig. S9). The CP build-up curve (Fig. 3B) shows that with a short contact time (<100 μs), the CP intensity increases rapidly and reaches a maximum at about 90 to 100 μs. With longer contact times, the signal decreases significantly. This CP behavior resembles the oxygen ions at Brønsted acid sites in acidic zeolites (23), as well as hydroxyl oxygen species in layered double hydroxides (24), indicating that this O species is directly bound to H. 1H-17O TRAPDOR NMR was also used to investigate the 1H-17O dipolar coupling on the surface of ceria nanoparticles (fig. S10). Significant TRAPDOR fraction can be observed at a rather short 17O irradiation time of about 100 μs, and this value reaches maximum at about 140 μs, similar to the observation in acidic zeolite HY (23), and again this result suggests that H is directly connected to O. Furthermore, the maximum TRAPDOR effect of ~23% shows the 17O isotopic molar percentage among the surface hydroxyl groups, indicating highly efficient 17O isotopic labeling of the surface of ceria nanoparticles.


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)

Double-resonance solid-state NMR data of ceria nanoparticles.(A, top to bottom) 17O MAS NMR spectra of ceria nanoparticles adsorbed with 17O water followed by thermal treatment under vacuum at 373 K at 14.1 and 9.4 T; difference spectrum in 17O-1H REDOR experiments; and 1H-17O CP MAS NMR spectrum with a contact time of 100 μs. (B) The peak intensity in CP MAS NMR experiment as a function of contact time. Spectra were obtained at both 9.4 and 14.1 T under MAS rates of 13 to 14 kHz. One thousand to 40,000 scans were averaged, and recycle delays from 0.2 to 1 s were used. Asterisks denote spinning sidebands.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 3: Double-resonance solid-state NMR data of ceria nanoparticles.(A, top to bottom) 17O MAS NMR spectra of ceria nanoparticles adsorbed with 17O water followed by thermal treatment under vacuum at 373 K at 14.1 and 9.4 T; difference spectrum in 17O-1H REDOR experiments; and 1H-17O CP MAS NMR spectrum with a contact time of 100 μs. (B) The peak intensity in CP MAS NMR experiment as a function of contact time. Spectra were obtained at both 9.4 and 14.1 T under MAS rates of 13 to 14 kHz. One thousand to 40,000 scans were averaged, and recycle delays from 0.2 to 1 s were used. Asterisks denote spinning sidebands.
Mentions: Interactions between ceria nanoparticles and water were also investigated in a reverse way, through the adsorption of 17O-enriched water on nonenriched ceria nanoparticles (preheated at 573 K). On adding H217O dropwise to ceria nanoparticles at room temperature, weak peaks at about 1040, 877, and 270 ppm appeared (Fig. 2B) in addition to the broad peak at about 32 ppm from adsorbed water. These peaks dominated the spectra after the sample was dried under vacuum at 373 K. Similar spectra were obtained by introducing H217O to nonenriched ceria nanoparticles calcined at 573 K on a vacuum line (Fig. 2B). The broad resonance at 270 ppm can be tentatively assigned to the hydroxyl groups (Ce4+-17OH) observed previously by Fourier transform infrared (20) on the surface of ceria nanoparticles on the basis of the low chemical shift. 17O-1H double-resonance NMR techniques [in this case, cross polarization (CP) (21) and rotational echo double resonance (REDOR) (22)] were used to confirm this assignment because they can be used to select the 17O resonances of oxygen ions close to protons: they measure the heteronuclear dipolar coupling (that is, between 17O and 1H), a function of distance between 17O and 1H. As seen from Fig. 3A, both the REDOR difference spectrum and CP spectra only show one signal at 270 ppm, confirming that this resonance comes from oxygen ions in close proximity to proton. The NMR parameters of this species, including chemical shift (δiso) and quadrupolar product [PQ = CQ(1 + ηQ2/3)1/2], were extracted by calculating the frequency change of the center of gravity of the signal at different external fields (300 ppm at 14.1 T versus 270 ppm at 9.4 T). The obtained values (δiso = 325 ppm, PQ = 5.1 MHz) are also supported by the DFT calculation results (fig. S8 and table S5). It is clear that the surface hydroxyl groups are associated with much larger CQ than the less coordinated surface oxygen species, and this is in agreement with the nutation curve (fig. S9). The CP build-up curve (Fig. 3B) shows that with a short contact time (<100 μs), the CP intensity increases rapidly and reaches a maximum at about 90 to 100 μs. With longer contact times, the signal decreases significantly. This CP behavior resembles the oxygen ions at Brønsted acid sites in acidic zeolites (23), as well as hydroxyl oxygen species in layered double hydroxides (24), indicating that this O species is directly bound to H. 1H-17O TRAPDOR NMR was also used to investigate the 1H-17O dipolar coupling on the surface of ceria nanoparticles (fig. S10). Significant TRAPDOR fraction can be observed at a rather short 17O irradiation time of about 100 μs, and this value reaches maximum at about 140 μs, similar to the observation in acidic zeolite HY (23), and again this result suggests that H is directly connected to O. Furthermore, the maximum TRAPDOR effect of ~23% shows the 17O isotopic molar percentage among the surface hydroxyl groups, indicating highly efficient 17O isotopic labeling of the surface of ceria nanoparticles.

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