Proton semiconductor

The most successful sensitizers so far tested are complexes of Ru(II) with various derivatives of 2,2 bipyridine, e.g. 2,2 -bipyridine 4,4 -dicarboxylic acid (L). The Ru(II)L3 complex is adsorbed from an aqueous solution of suitable pH value to oxidic semiconductors via electrostatic bonds between —COO- groups of the ligands and the positively charged (protonized) semiconductor surface. 

Morowitz, H. J. Proton semiconductors and energy transduction in biological systems. American Journal of Physiology, 235 (1978), R99-114. 

Electrical measurements of ice are diflBcult to interpret because of polarization effects, surface conductivity, injection of defects and/or impurity atoms from sandwich electrodes, diffusion effects, differential ion incorporation, and concentration gradients due to nonsteady state impurity distribution. Theories formulated for pure ice and for ice doped with HF (KF and CsF) in terms of ion states and valence defects, qualitatively account for experimental data, although the problem of the majority and minority carriers in doped ice, as a function of concentration and temperature, requires further examination. The measurements on ice prepared from ionic solutes other than HF, KF, and CsF are largely unexplained. An alternative approach that treats ice as a protonic semiconductor accounts for results obtained for both the before-named impurities as well as ammonia and ammonium fluoride. 

The most significant result which comes out of this work is that Mg(OH)2, Ca(OH)2 and Al(OH)3 may be protonic semiconductors that can be described by highly localized protons occupying a valence band, VB, and the presence of a conduction band, CB, providing for proton delocalization. Between VB and CB lie intermediate levels, corresponding to the vibrationally excited states of the O-H oscillator, separated by Si 0.4 eV. The gap values are 2.0-2.1 eV for Mg(OH)2,2.2 eV for the more basic Ca(OH)2 and 1.6-1.7 eV for the more acidic Al(OH)3. 

Because there are two changes ia material composition near the active region, this represents a double heterojunction. Also shown ia Figure 12 is a stripe geometry that confines the current ia the direction parallel to the length of the junction. This further reduces the power threshold and makes the diffraction-limited spreading of the beam more symmetric. The stripe is often defined by implantation of protons, which reduces the electrical conductivity ia the implanted regions. Many different stmctures for semiconductor diode lasers have been developed. 

The different types of quinones active in photosynthesis are being used as electron acceptors in solar cells. The compounds such as Fd and NADP could also be used as electron/proton acceptors in the photoelectrochemical cells. Several researchers have attempted the same approach with a combination of two or more solid-state junctions or semiconductor-electrolyte junctions using bulk materials and powders. Here, the semiconductors can be chosen to carry out either oxygen- or hydrogen-evolving photocatalysis based on the semiconductor electronic band structure. 

At present, several stable photocatalytic systems for production of hydrogen from water and organic compounds are made of semiconducting oxides and suitable proton reducing catalyzer. An efficient electron transfer between inorganic semiconductor and bacterial hydrogenase was shown to result in hydrogen photoproduction. 

Acceptor doping in perovskite oxides gives materials with a vacancy population that can act as proton conductors in moist atmospheres (Section 6.9). In addition, the doped materials are generally p-type semiconductors. This means that in moist atmospheres there is the possibility of mixed conductivity involving three charge carriers (H+, O2-, and h ) or four if electrons, e, are included. 

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