ABSTRACT
Ice surfaces are closely relevant to many physical and chemical properties, such as melting, freezing, friction, gas uptake and atmospheric reaction1-8. Despite extensive experimental and theoretical investigations9-17, the exact atomic structures of ice interfaces remain elusive owing to the vulnerable hydrogen-bonding network and the complicated premelting process. Here we realize atomic-resolution imaging of the basal (0001) surface structure of hexagonal water ice (ice Ih) by using qPlus-based cryogenic atomic force microscopy with a carbon monoxide-functionalized tip. We find that the crystalline ice-Ih surface consists of mixed Ih- and cubic (Ic)-stacking nanodomains, forming 19 × 19 periodic superstructures. Density functional theory reveals that this reconstructed surface is stabilized over the ideal ice surface mainly by minimizing the electrostatic repulsion between dangling OH bonds. Moreover, we observe that the ice surface gradually becomes disordered with increasing temperature (above 120 Kelvin), indicating the onset of the premelting process. The surface premelting occurs from the defective boundaries between the Ih and Ic domains and can be promoted by the formation of a planar local structure. These results put an end to the longstanding debate on ice surface structures and shed light on the molecular origin of ice premelting, which may lead to a paradigm shift in the understanding of ice physics and chemistry.
ABSTRACT
The permeability and selectivity of biological and artificial ion channels correlate with the specific hydration structure of single ions. However, fundamental understanding of the effect of ion-ion interaction remains elusive. Here, via non-contact atomic force microscopy measurements, we demonstrate that hydrated alkali metal cations (Na+ and K+) at charged surfaces could come into close contact with each other through partial dehydration and water rearrangement processes, forming one-dimensional chain structures. We prove that the interplay at the nanoscale between the water-ion and water-water interaction can lead to an effective ion-ion attraction overcoming the ionic Coulomb repulsion. The tendency for different ions to become closely packed follows the sequence K+ > Na+ > Li+, which is attributed to their different dehydration energies and charge densities. This work highlights the key role of water molecules in prompting close packing and concerted movement of ions at charged surfaces, which may provide new insights into the mechanism of ion transport under atomic confinement.
ABSTRACT
The nature of hydrated proton on solid surfaces is of vital importance in electrochemistry, proton channels, and hydrogen fuel cells but remains unclear because of the lack of atomic-scale characterization. We directly visualized Eigen- and Zundel-type hydrated protons within the hydrogen bonding water network on Au(111) and Pt(111) surfaces, using cryogenic qPlus-based atomic force microscopy under ultrahigh vacuum. We found that the Eigen cations self-assembled into monolayer structures with local order, and the Zundel cations formed long-range ordered structures stabilized by nuclear quantum effects. Two Eigen cations could combine into one Zundel cation accompanied with a simultaneous proton transfer to the surface. Moreover, we revealed that the Zundel configuration was preferred over the Eigen on Pt(111), and such a preference was absent on Au(111).
ABSTRACT
Although thermal conductivity gas analyzers are ubiquitous in industry, shrinking the sensing unit to a microscopic scale is rarely achieved. Since heat transfer between a metal nanoparticle and its ambient gas changes the temperature, refractive index, and density of the gaseous surrounding, one may tackle the problem using a single nanoparticle's photothermal effect. Upon heating by a 532 nm laser, a single gold nanoparticle transfers heat to the surrounding gas environment, which results in a change in the photothermal polarization of a 633 nm probe laser. The amplitude of the photothermal signal correlates directly with the concentration of binary gas mixture. In He/Ar, He/N2, He/air, and H2/Ar binary gas mixtures, the signal is linearly proportional to the He and H2 molar concentrations up to about 10%. The photothermal response comes from the microscopic gaseous environment of a single gold nanoparticle, extending from the nanoparticle roughly to the length of the gas molecule's mean free path. This study points to a way of sensing binary gas composition in a microscopic volume using a single metal nanoparticle.
ABSTRACT
OBJECTIVE: To study the protective effects of the cream of the total flavonoids from Oxytropis falcata on the destructed skin of mice induced by moderate-wave ultraviolet (UVB) irradiation. METHODS: Dorsal skin of Wistar mice were treated with the cream of the total flavonoids from Oxytropis falcata and then irradiated with UVB in the dosage of 5 min once a day for one week. The tissue of skin was pathological diagnosed and the activities or contents of superoxide dismutase (SOD), malondialdehyde (MDA), hydroxyproline (Hyp), glutathione peroxidease (GSH-Px), glutathione (GSH), glutathion-s-transferase (GST), catalase (CAT) and hydroxy radical (*OH) were determined with chromatometry. RESULTS: The ultraviolet protective effects of the cream could be observed with appearance and pathology examine. The cream could increase the activities of SOD (P < 0.001), GSH-Px (P < 0.001), GST (P < 0.05) and CAT (P < 0.01), raise the content of Hyp (P < 0.001) significantly. The cream could also decrease the contents of MDA and *OH (P < 0.001), and the activities of GSH significantly (P < 0.001). CONCLUSION: The cream of the total flavonoids from Oxytropis falcata has protective effect on the destructed skin of mice induced by moderate-wave ultraviolet (UVB) irradiation.
