Distinct water activation on polar/non-polar facets of ZnO nanoparticles

Facet-dependent transformation and toxicity of nanoscale zinc oxide in the synthetic saliva

The nanoscale zinc oxide (n-ZnO) was used in food packages due to its superior antibacterial activity, resulting in potential intake of n-ZnO through the digestive system, wherein n-ZnO interacted with saliva. In recent, facet engineering, a technique for controlling the exposed facets, was applied to n-ZnO, whereas risk of n-ZnO with specific exposed facets in saliva was ignored. ZnO nanoflakes (ZnO-0001) and nanoneedles (ZnO-1010) with the primary exposed facets of {0001} and {1010} respectively were prepared in this study, investigating stability and toxicity of ZnO-0001 and ZnO-1010 in synthetic saliva. Both ZnO-0001 and ZnO-1010 partially transformed into amorphous Zn3(PO4)2 within 1 hr in the saliva even containing orgnaic components, forming a ZnO-Zn3(PO4)2 core-shell structure. Nevertheless, ZnO-1010 relative to ZnO-0001 would likely transform into Zn3(PO4)2, being attributed to superior dissolution of {1010} facet due to its lower vacancy formation energy (1.15 eV) than {0001} facet (3.90 eV). The toxicity of n-ZnO to Caco-2 cells was also dependent on the primary exposed facet; ZnO-0001 caused cell toxicity through oxidative stress, whereas ZnO-1010 resulted in lower cells viability than ZnO-0001 through oxidative stress and membrane damage. Density functional theory calculations illustrated that ·O2- was formed and released on {1010} facet, yet O22- instead of ·O2- was generated on {0001} facet, leading to low oxidative stress from ZnO-0001. All findings demonstrated that stability and toxicity of n-ZnO were dependent on the primary exposed facet, improving our understanding of health risk of nanomaterials.

Effect of polar/non-polar facets on the transformation of nanoscale ZnO in simulated sweat and potential impacts on the antibacterial activity

The use of nanoscale zinc oxide (n-ZnO) in the personal care products would cause interactions between n-ZnO and human sweat. Facet engineering has been applied to n-ZnO to improve its activity. Nevertheless, it is not clear whether the exposed facet would affect transformation of n-ZnO in sweat. Herein, we prepared ZnO nanoneedles with the dominant (1010) non-polar facet (i.e., ZnO-1010) and ZnO nanoflakes with the dominant (0001) polar facet (i.e., ZnO-0001), respectively. We found that n-ZnO can undergo chemical transformation in the simulated sweat within 168 h or 24 h, transforming into amorphous materials and Zn₃(PO₄)₂0.4 H₂O and/or Na(ZnPO₄)·H₂O. Given the rate constant (e.g., 0.093 h^-1 for ZnO-0001 vs. 0.033 h^-1 for ZnO-1010) of ZnO depletion and components of the precipitate from the simulated sweat, nevertheless, the transformation is highly dependent on the dominant exposed facet of n-ZnO. The ZnO-0001 relative to ZnO-1010 would likely undergo chemical transformation, demonstrating that the (0001) polar facet compared to (1010) non-polar facet had a superior activity to the dihydrogen phosphate anions in the simulated sweat, which is supported by density functional theory calculations. The chemical transformation can affect the antibacterial activity of n-ZnO to E. coli, moderating the toxicity due to a great decrease in the concentration of the dissolved zinc. In total, our findings provided insights into the facet-dependent transformation for n-ZnO in the simulated sweat, improving our understanding of potential risk of n-ZnO.

Elucidating the Cooperative Roles of Water and Lewis Acid-Base Pairs in Cascade C-C Coupling and Self-Deoxygenation Reactions

Water plays pivotal roles in tailoring reaction pathways in many important reactions, including cascade C-C bond formation and oxygen elimination. Herein, a kinetic study combined with complementary analyses (DRIFTS, isotopic study, ¹H solid-state magic angle spinning nuclear magnetic resonance) and density functional theory (DFT) calculations are performed to elucidate the roles of water in cascade acetone-to-isobutene reactions on a Zn _x Zr _y O _z mixed metal oxide with balanced Lewis acid-base pairs. Our results reveal that the reaction follows the acetone-diacetone alcohol-isobutene pathway. Isobutene is produced through an intramolecular rearrangement of the eight-membered ring intermediate formed via the adsorption of diacetone alcohol on the Lewis acid-base pairs in the presence of cofed water. OH adspecies, formed by the dissociative adsorption of water on the catalyst surface, were found to distort diacetone alcohol's hydroxyl functional group toward its carbonyl functional group and facilitate the intramolecular rearrangement of diacetone alcohol to form isobutene. In the absence of water, diacetone alcohol binds strongly to the Lewis acid site, e.g., at a Zr⁴⁺ site, via its carbonyl functional group, leading to its dramatic structural distortion and further dehydration reaction to form mesityl oxide as well as subsequent polymerization reactions and the formation of coke. The present results provide insights into the cooperative roles of water and Lewis acid-base pairs in catalytic upgrading of biomass to fuels and chemicals.

Direct Catalytic Conversion of Ethanol to C5+ Ketones: Role of Pd-Zn Alloy on Catalytic Activity and Stability

Ethanol can be used as a platform molecule for synthesizing valuable chemicals and fuel precursors. Direct synthesis of C₅₊ ketones, building blocks for lubricants and hydrocarbon fuels, from ethanol was achieved over a stable Pd-promoted ZnO-ZrO₂ catalyst. The sequence of reaction steps involved in the C₅₊ ketone formation from ethanol was determined. The key reaction steps were found to be the in situ generation of the acetone intermediate and the cross-aldol condensation between the reaction intermediates acetaldehyde and acetone. The formation of a Pd-Zn alloy in situ was identified to be the critical factor in maintaining high yield to the C₅₊ ketones and the stability of the catalyst. A yield of >70 % to C₅₊ ketones was achieved over a 0.1 % Pd-ZnO-ZrO₂ mixed oxide catalyst, and the catalyst was demonstrated to be stable beyond 2000 hours on stream without any catalyst deactivation.

Room temperature sintering of polar ZnO nanosheets: II-mechanism

In a previous work by the authors (A. Fernández-Pérez el al., Room temperature sintering of polar ZnO nanosheets: I-evidence, 2017, DOI: 10.1039/C7CP02306E), polar ZnO nanosheets were stored at room temperature under different atmospheres and the evolution of their textural and crystal properties during storage was followed. It was found that the specific surface area of the nanosheets drastically decreased during storage, with a loss of up to 75%. The ZnO crystals increased in size mainly through the partial merging of their polar surfaces at the expense of narrow mesoporosity, in a process triggered by the action of moisture, oxygen and, in their absence, by light. In the present work, a set of spectroscopic techniques (FTIR, Raman and XPS) has been used in an attempt to unravel the mechanism behind this spontaneous sintering process. The mechanism starts with the molecular adsorption of water, which takes place on Zn atoms close to oxygen vacancies on the (100) surface, where H₂O dissociates to form two hydroxyl groups and to heal one oxygen vacancy. This process triggers the room temperature migration of Zn interstitials towards the outer surface of the polar region. What were previously interstitial Zn atoms now gradually occupy the mesopores, with interstitial oxygen being used to build up the O sublattice until total occupancy of the narrow mesoporosity is achieved.

Room temperature sintering of polar ZnO nanosheets: I-evidence

Unambiguous evidence of the spontaneous loss of surface area at room temperature in polar ZnO.