John G. Mavroides

Photoelectrolysis of water in cells with SrTiO3 anodes

The photoelectrolysis of water in cells with SrTiO3 anodes has been confirmed. The maximum external quantum efficiency at zero bias voltage is 10% (at hν=3.8 eV), about an order of magnitude higher than the maximum value obtained with anodes of TiO2, the only other material so far known to catalyze photoelectrolysis. In agreement with the energy‐level model proposed previously, the efficiency is increased because the band bending at the anode surface is about 0.2 eV larger for SrTiO3 than for TiO2, as a result of the smaller electron affinity of SrTiO3.

Photoelectrolysis of water in cells with TiO2 anodes

Abstract The photoelectrolysis of water has been investigated by experiments on cells consisting of an illuminated nTiO 2 (rutile) anode, an aqueous electrolyte, and a platinized-Pt cathode. It has been found that such cells operate either in the photogalvanic mode (no H 2 evolved) or in the photoelectrolytic mode (H 2 evolved at the cathode by decomposition of water), depending on whether or not the electrolyte surrounding the cathode contains dissolved oxygen. In both cases, current flows through the external circuit and O 2 is evolved at the anode. For operation in the photogalvanic mode, maximum values of 80–85% for the external quantum efficiency (η) for current production have been measured at h v ≈4 eV with both single-crystal and polycrystalline TiO 2 anodes. Similar results have been obtained in preliminary experiments with SrTiO 3 anodes. The internal quantum efficiencies, corrected for reflection and absorption losses, are close to 100%, indicating that the band bending in TiO 2 under photogalvanic conditions is sufficient to separate the electron-hole pairs generated by photon absorption and also that the oxygen over-voltage for charge transfer at the semiconductor-electrolyte interface is negligible for illuminated anodes. For operation in the photoelectrolytic mode, η is only 1–2% if the anode and cathode are shorted together, but the efficiency can be greatly increased by applying a bias voltage. By using a photogalvanic TiO 2 -Pt cell to supply this voltage, it has been possible to obtain η values of ∼20%, computed on the basis of the total number of photons incident on the anodes of both cells. All the observations can be given a straightforward explanation in terms of the energy levels of the electrodes and the electrolyte.

Magnetoreflection studies on the band structure of bismuth-antimony alloys☆

Abstract We present magnetoreflection results in the BiSb alloys which indicate that the bismuth L-point energy gap changes with alloying; these results suggest an experimentally consistent pattern for the variation of the energy bands at the L-and T-points in the Brillouin zone for the BiSbAs system in which spin-orbit interaction appears to be the primary variable.

Low resistance Pd/Ge/Au and Ge/Pd/Au ohmic contacts to n‐type GaAs

We have fabricated Pd/Ge/Au and Ge/Pd/Au sintered ohmic contacts on n‐type GaAs. These contacts have specific resistances similar to those of conventional Ni/Ge/Au alloyed ohmic contacts, but their surfaces are much smoother and their edges are well defined. The fabrication procedure is compatible with GaAs technology, and the sintered contacts are thermally stable. The properties of these low resistance contacts are insensitive to the sintering temperature and to the thickness of the Pd and Ge layers or their order of deposition.

Band structure of HgTe and HgTe-CdTe alloys

Abstract The gray tin band model of Groves and Paul, which is extended to include overlap of the valence and conduction bands, is applied to HgTe and Cd x Hg 1−x Te alloys. Theoretical and experimental evidence for the model is presented, and band parameters deduced from experimental data are given.

The fermi surface of graphite

Recent magnetoreflection measurements in pyrolytic graphite have been interpreted using the magnetic energy levels obtained from the McClure-lnoue secular equation and the appropriate selection rules for interband transitions. Combining these results with those of the de Haas - van Alphen effect, the band parameters of the Slonczewski-Weiss model have been evaluated and the Fermi surface determined. The magnetoreflection experiment indicates considerable warping of the Fermi surface, particularly for holes. Further experiments to determine this warping more precisely are discussed.

Electrode materials for the photoelectrolysis of water

Abstract There has recently been a renewed interest in photoelectrochemical processes at semiconductor-electrolyte interfaces for possible applications in solar conversion. In particular, the photoelectrolysis of water by sunlight in electrochemical cells to produce the fuel H 2 has resulted in considerable research. In this paper, the concepts of photoelectrolysis are used as a guide for deriving the physical and chemical properties necessary for materials to be efficient electrodes in such cells. Presently available materials are considered in this context along with prospects for the future.

Restrahlen reflection in HgTe

Abstract We have observed the restrahlen reflection at λ = 85 μ . By fitting the experimental points with a classical Lorentz oscillator, it is found that the fundamental lattice absorption frequency ν 0 = 3.45 × 10 12 , the optical dielectric constant ϵ 0 = 14 and the effective charge q ∗ = 0.6 e .

Elastic constants of HgTe

Abstract Using the ultrasonic pulse technique, the room temperature elastic constants of HgTe were found to be, in units of 1011 dynes/cm2: c11 = 5.08, c12 = 3.58, and c44 = 2.05. From these values, a fundamental lattice absorption frequency ωo = 1.87 × 1013 and a Debye characteristic temperature at absolute zero, ϑo = 105°K have been calculated.

Spin-orbit interaction in graphite

Abstract Recent measurements of the magnetoreflection and of the de Haas-van Alphen effect have made possible a more accurate determination of the band parameters of pyrolytic graphite. In principle, these experiments could provide a more direct measurement of the small spin-orbit interaction in graphite. This interaction lifts the degeneracy of the E3 bands everywhere along the zone edge and gives rise to a band gap near point K which could be deduced from analysis of the magnetoreflection experiment. When the degeneracy of the energy bands at point H is lifted, two extremal Fermi surface areas are obtained, thus predicting two low frequency de Haas-van Alphen oscillations.