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).
The detailed understanding of various underlying processes at liquid/solid interfaces requires the development of interface-sensitive and high-resolution experimental techniques with atomic precision. In this perspective, we review the recent advances in studying the liquid/solid interfaces at atomic level by electrochemical scanning tunneling microscope (EC-STM), non-contact atomic force microscopy (NC-AFM), and surface-sensitive vibrational spectroscopies. Different from the ultrahigh vacuum and cryogenic experiments, these techniques are all operated in situ under ambient condition, making the measurements close to the native state of the liquid/solid interface. In the end, we present some perspectives on emerging techniques, which can defeat the limitation of existing imaging and spectroscopic methods in the characterization of liquid/solid interfaces.
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/N
2
, He/air, and H
2
/Ar binary gas mixtures, the signal is
linearly proportional to the He and H
2
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.
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