Specific ion effects ranking in the Hofmeister sequence are ubiquitous in biochemical, industrial, and atmospheric processes. In this experimental study specific ion effects inexplicable by the classical DLVO theory have been investigated at curved water-metal interfaces of gold nanoparticles synthesized by a laser ablation process in liquid in the absence of any organic stabilizers. Notably, ion-specific differences in colloidal stability occurred in the Hückel regime at extraordinarily low salinities below 50 μM, and indications of a direct influence of ion-specific effects on the nanoparticle formation process are found. UV-vis, zeta potential, and XPS measurements help to elucidate coagulation properties, electrokinetic potential, and the oxidation state of pristine gold nanoparticles. The results clearly demonstrate that stabilization of ligand-free gold nanoparticles scales proportionally with polarizability and antiproportionally with hydration of anions located at defined positions in a direct Hofmeister sequence of anions. These specific ion effects might be due to the adsorption of chaotropic anions (Br(-), SCN(-), or I(-)) at the gold/water interface, leading to repulsive interactions between the partially oxidized gold particles during the nanoparticle formation process. On the other hand, kosmotropic anions (F(-) or SO4(2-)) seem to destabilize the gold colloid, whereas Cl(-) and NO3(-) give rise to an intermediate stability. Quantification of surface charge density indicated that particle stabilization is dominated by ion adsorption and not by surface oxidation. Fundamental insights into specific ion effects on ligand-free aqueous gold nanoparticles beyond purely electrostatic interactions are of paramount importance in biomedical or catalytic applications, since colloidal stability appears to depend greatly on the type of salt rather than on the amount.

The energy released in low-energy chemisorption or physisorption of molecules on metal surfaces is usually expected to be dissipated by surface vibrations (phonons). Theoretical descriptions of competing electronic excitations are incomplete, and experimental observation of excited charge carriers has been difficult except at energies high enough to eject electrons from the surface. We observed reaction-induced electron excitations during gas interactions with polycrystalline silver for a variety of species with adsorption energies between 0.2 and 3.5 electron volts. The probability of exciting a detectable electron increases with increasing adsorption energy, and the measured time dependence of the electron current can be understood in terms of the strength and mechanism of adsorption.

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Hot electrons and holes created at Ag and Cu surfaces by adsorption of thermal hydrogen and deuterium atoms have been measured directly with ultrathin metal film Schottky diode detectors on Si(111). When the metal surface is exposed to these atoms, charge carriers are excited at the surface, travel ballistically toward the interface, and have been detected as a chemicurrent in the diode. The current decreases with increasing exposure and eventually reaches a constant value at the steady-state coverage. This is the first direct evidence of nonadiabatic energy dissipation during adsorption at transition metal surfaces. [S0031-9007(98) PACS numbers: 82.20. Rp, 34.50.Dy, 79.20.Rf, 82.65.My The pathways of energy dissipation in exothermic reactions at metal surfaces are of fundamental interest but still poorly understood. Bond formation energies of up to several eV are transferred into the substrate. Since bulk phonon energies are typically 2 orders of magnitude smaller, nonadiabatic excitations of electron-hole (e-h) pairs may be a significant alternative to multiple phonon creation [1][2][3][4]. With surface reactions at thermal collision energies, there are a few examples of energy transfer into the electronic system accompanied by light emission (chemiluminescence) [5] and exoelectron ejection [6]. These effects are observed only with the exothermic adsorption of electronegative molecules on reactive metal surfaces. In addition, exoelectron emission requires that the metal has a low work function. Heretofore, there has been no direct experimental evidence for adsorptioninduced e-h pair excitations at transition metal surfaces. Although the maximum energy of any hot charge carriers is smaller than the metal work function, precluding exoelectron emission, the energy may be sufficiently large to enable the charge carriers to be collected by crossing a smaller potential barrier.We demonstrate in the present study that the direct detection of chemisorption-induced e-h pairs is feasible using the Schottky barrier of a transition metal-semiconductor diode detector. We chose the adsorption of atomic hydrogen on Ag and Cu film surfaces as model systems. These metals exhibit high reactivity to atomic hydrogen but negligible dissociative adsorption of H 2 [7]. The formation energy of the H-metal bond is large, about 2.5 eV in both cases [7]. To detect the hot charge carriers, we designed a sensor consisting of a large-area metal-semiconductor contact with an ultrathin metal film. The proposed mechanism of current production in the sensor is illustrated in Fig. 1(a) for the case of hot electrons. The transition metal film is evaporated on n-type Si forming a diode with a Schottky barrier, F. Figure 1(a) shows the Fermi level, E F , the conduction band minimum (CBM) and the valence band maximum (VBM). If the exothermic chemisorption of H atoms creates e-h pairs, hot electrons may travel ballistically through the film and cross the barrier. They can be detected as a current which we hereafter call a chemicurrent. Similarly...

The electronic properties and the electron affinities of AlxGa1−xN(0001) surfaces were investigated by ultraviolet photoemission spectroscopy (UPS) over the whole composition range. The samples were prepared by N-ion sputtering and annealing. Surface cleanliness and stoichiometry were monitored with x-ray photoemission spectroscopy. Samples with high aluminum content showed traces of oxygen which could not be removed by further cleaning cycles. However, we have evidence that the oxygen is located in the bulk and not at the surface. From the UP spectra the ionization energies and electron affinities as a function of composition x were determined. A decrease in electron affinity with increasing aluminum content was found, but the electron affinity remains positive for all x. Thus, earlier predictions of negative electron affinity for high aluminum content were not confirmed.

Published by the AIP PublishingArticles you may be interested in Improved performance of Pd/WO3/SiC Schottky-diode hydrogen gas sensor by using fluorine plasma treatment Appl. Phys. Lett. 107, 073506 (2015)

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