Surface differences of oxide nanocrystals determined by geometry and exogenously coordinated water molecules

Abstract

Both atomic geometry and the influence of surroundings (e.g., exogenously coordinated water) are key issues for determining the chemical environment of oxide surfaces, whereas the latter is usually ignored and should be considered in future studies.

Double-resonance17O NMR experiments reveal unique configurational information for surface organometallic complexes

“…21 This procedure is general and can be applied to any oxide support. 22–26 At most, this procedure can lead to an enrichment at half the enrichment level of the water at each step, but in practice, we observe roughly a quarter (Fig. 1b).…”

Emerging Applications of 17O Solid-State NMR Spectroscopy for Catalytic Oxides

“…Longitudinal relaxation times ( T 1 s) obtained from variable temperature NMR experiments show a low activation energy of 0.17 eV for surface water motion on (111) facets of ceria nanorods, indicating that this motion is facile and there is only weak interaction between adsorbed water molecules and ceria surface . These dynamic processes, along with surface geometry variation due to water, are associated with the variation of the observed 17 O NMR shifts of the surface oxygen species. , The exchanges of oxygen ions in ZnO nanocrystals with water can also be monitored by in situ 17 O MAS NMR, following the initial enrichment procedures . In addition to the surface species, these dynamic exchanges also involve species in the core of ZnO, which was not expected originally.…”

“…104−107 For example, the differences between the calculated 17 O NMR shifts and experimental results on oxide nanocrystals are usually less than 10 ppm. 60,61,97,108 At the same time, motion may explain the additional differences between the prediction and experimental data, which can be probed by using variable temperature NMR spectroscopy. 98,101,109 It should also be noted that 'DFT+U' is widely used in calculations of properties of many oxides, 110 and the choice of Hubbard U corrections may have a great impact on the NMR parameters.…”

“…98 These dynamic processes, along with surface geometry variation due to water, are associated with the variation of the observed 17 O NMR shifts of the surface oxygen species. 98,108 The exchanges of oxygen ions in ZnO nanocrystals with water can also be monitored by in situ 17 O MAS NMR, following the initial enrichment procedures. 62 In addition to the surface species, these dynamic exchanges also involve species in the core of ZnO, which was not expected originally.…”

“…GIPAW calculations generally give very good predictions of the NMR parameters, despite the fact that the DFT calculations are performed on static structures at 0 K while NMR experiments are mostly acquired at room temperature. − For example, the differences between the calculated 17 O NMR shifts and experimental results on oxide nanocrystals are usually less than 10 ppm. ,,, At the same time, motion may explain the additional differences between the prediction and experimental data, which can be probed by using variable temperature NMR spectroscopy. ,, It should also be noted that ‘DFT+ U ’ is widely used in calculations of properties of many oxides, and the choice of Hubbard U corrections may have a great impact on the NMR parameters. For example, while applying Hubbard U corrections on all the O 2p and Zn 3d orbitals for ZnO gives good predictions for most of the 17 O NMR parameters of almost all oxygen species in ZnO nanorods, the calculated C Q s for surface water and hydroxyl species (5.3–8.2 MHz) are much larger than experimental values (2.5 MHz) .…”