Electron-Hole Pair Creation at Ag and Cu Surfaces by Adsorption of Atomic Hydrogen and Deuterium

Abstract

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...

Unified description of H-atom–induced chemicurrents and inelastic scattering

“…This analysis is consistent with previous work suggesting that only the H-H recombination reaction on Au produces hot electrons (24) and chemicurrent (13). We also note that the H-adsorption-induced chemicurrents seen by Schottky diode experiments on Ag surfaces (7,11) were performed at a surface temperature (100-135 K) far lower than the H 2 recombinative desorption temperature (∼170 K) (25). Thus, there was no chance that this source of hot electrons could have been seen in those experiments.…”

“…However, a growing number of examples have been found where electronic and nuclear degrees of freedom are strongly coupled in violation of the BOA (2-6). H-atom interactions at metals offer a remarkable opportunity to test non-BOA theories against experiment, since H-adsorptioninduced chemicurrent experiments (7)(8)(9)(10)(11)(12)(13)(14) offer a direct measure of electronic excitation and H-atom inelastic scattering experiments (15) directly probe nuclear motion. In chemicurrent experiments, exothermic H interactions like adsorption and recombination produce hot electrons that pass over a potential barrier to be collected.…”

“…Hence, the magnitude of the chemicurrent is dependent on both the reaction-induced production of hot electrons and the likelihood of transmission over the barrier. To reduce uncertainties associated with barrier transmission, the ratio of hydrogen-and deuterium-induced chemicurrent is often measured--H-induced chemicurrents are typically two to five times larger than those from D atoms (7,(11)(12)(13). H-atom surface scattering experiments yield H-atom translational energy loss distributions.…”

Hydrogen-Etched TiO2−x as Efficient Support of Gold Catalysts for Water–Gas Shift Reaction

“…As shown in Figure 10a, since the work function of metal Au (Φ m = 4.8 eV) is smaller than that of semiconductor TiO 2 (Φ s = 5.1 eV) [70], the electrons can diffuse from the metal into the semiconductor when the two phases are in contact [71,72]. This electron transfer was called "hot-electron flow" [56,57,[73][74][75] or "chemicurrent" [76][77][78], which usually happened in exothermic catalytic reactions [56,57,[73][74][75] and low-energy reactions [74] or even nonthermal directions [75]. For example, it has been reported that electron excitation was also involved in atomic/molecular adsorption processes [73,77,78].…”

Hydrogen Oxidation-Driven Hot Electron Flow Detected by Catalytic Nanodiodes

“…1a. Non-catalytic metal-semiconductor diodes have been used to prove that hot electrons are generated in adsorption reactions in ultrahigh vacuum (UHV) at temperatures below 150 K [18][19][20] . By using catalytic metal-semiconductor diodes such as Pt/TiO 2 or Pt/GaN, it has also been possible to detect steady chemicurrent from catalytic CO oxidation at atmospheric pressure, between 400 K and 550 K 16,21 .…”