Modifying Surface Chemistry of Metal Oxides for Boosting Dissolution Kinetics in Water by Liquid Cell Electron Microscopy

Abstract

Dissolution of metal oxides is fundamentally important for understanding mineral evolution and micromachining oxide functional materials. In general, dissolution of metal oxides is a slow and inefficient chemical reaction. Here, by introducing oxygen deficiencies to modify the surface chemistry of oxides, we can boost the dissolution kinetics of metal oxides in water, as in situ demonstrated in a liquid environmental transmission electron microscope (LETEM). The dissolution rate constant significantly increases by 16-19 orders of magnitude, equivalent to a reduction of 0.97-1.11 eV in activation energy, as compared with the normal dissolution in acid. It is evidenced from the high-resolution TEM imaging, electron energy loss spectra, and first-principle calculations where the dissolution route of metal oxides is dynamically changed by local interoperability between altered water chemistry and surface oxygen deficiencies via electron radiolysis. This discovery inspires the development of a highly efficient electron lithography method for metal oxide films in ecofriendly water, which offers an advanced technique for nanodevice fabrication.

Assessment of oxide nanoparticle stability in liquid phase transmission electron microscopy

“…However, because they are easily reducible, it is important to also consider their sub-oxides such as Cu2O, FeO and Ce2O3. As can be seen in Table 2, these sub-oxides actually have a negative ΔG 0 H. This is also in agreement with the observations of Lu et al [55], in which they report that electron beam induced dissolution of cerium oxide becomes extremely fast only after the Ce2O3 phase is observed. The sub-oxides of TiO2 (Ti2O3 and TiO) on the other hand have a positive ΔG 0 H, indicating TiO2 should be stable according to this correlation, which is indeed the case.…”

“…Another difference between the stable and unstable oxides in this study is that the stable oxides are all reducible, relatively easily releasing oxygen atoms, which could also be used to explain these observations. However, a recent study showed that Fe2O3, CeO2 and CuO, which are also reducible oxides, were unstable under electron beam irradiation in the presence of water [55]. Seeing that these three oxides also have a positive ΔG 0 H, according to our hypothesis they should be stable.…”

“…Although the oxides that appeared stable in LP-TEM also have a very low solubility, the oxide with the highest solubility of all is SiO2, which proved to be much more stable than either Al2O3 or MgO, both of which exhibit lower solubility than SiO2. Furthermore, it has been argued that in aqueous environments subjected to intense ionizing radiation, local supersaturation could be present [55], suggesting that solubility is not the major cause for the observed differences in stability.…”

“…Recent investigations on iron (hydr)oxide [51] and silicon dioxide [52][53][54] have shown that oxide materials can also suffer from electron beam induced destabilization in a liquid environment. In addition, Lu et al [55] investigated dissolution behavior of several oxides in LP-TEM. They observed a significant increase in dissolution rate of the oxides in the presence of the electron beam, which was attributed to the electron beam induced formation of oxygen vacancies.…”

In-situ observation of radiation physics and chemistry of nanostructured cerium oxide in water

“…Irradiation induced enhancement of the fraction of Ce 3+ species in our CeO 2 particles is another mechanism, complementary to and distinct from standard reductive dissolution, where the Ce-reduction originates by water. Other most recent work have proposed alternatively either solid Ce(OH) 3 [48] species as end product or solid Ce 2 O 3 species [53] as temporary intermediate. Our reported high dissolution rates [31] are however compatible with early stage persistance of CeO 2 , and reproduced in [53].…”