Zinc nickel oxide semiconductor

We normally define the energy level of electrons in a solid in terms of the Fermi level, eF, which is essentially equivalent to the electrochemical potential of electrons in the solid. In the case of metals, the Fermi level is equal to the highest occupied level of electrons in the partially filled frontier band. In the case of semiconductors of covalent and ionic solids, by contrast, the Fermi level is situated within the band gap where no electron levels are available except for localized ones. A semiconductor is either n-type or p-type, depending on its impurities and lattice defects. For n-type semiconductors, the Fermi level is located close to the conduction band edge, while it is located close to the valence band edge for p-type semiconductors. For examples, a zinc oxide containing indium as donor impurities is an n-type semiconductor, and a nickel oxide containing nickel ion vacancies, which accept electrons, makes a p-type semiconductor. In semiconductors, impurities and lattice defects that donate electrons introduce freely mobile electrons in the conduction band, and those that accept electrons leave mobile holes (electron vacancies) in the valence band. Both the conduction band electrons and the valence band holes contribute to electronic conduction in semiconductors. 

Figure 13.19 The conversion of insulating oxides into semiconductors, (a) (i) Nickel oxide (NiO) doped with hthium oxide (Li20), making it a p-t)fpe semiconductor, and (ii) the energy-band structure of Li+-doped NiO. (b) (i) Zinc oxide (ZnO) doped with gallium oxide (Ga203), making it an n-type semiconductor, and (ii) the energy-band struemre of Ga -doped ZnO...

Because of the potential importance for industrial-scale catalysis, we decided to check (i) whether an influence of a semiconductor support on a metal catalyst was present also if the metal is not spread as a thin layer on the semiconductor surface but rather exists in form of small particles mixed intimately with a powder of the semiconductor, and (ii) whether a doping effect was present even then. To this end the nitrates of nickel, zinc (zinc oxide is a well-characterized n-type semiconductor) and of the doping element gallium (for increased n-type doping) or lithium (for decreased n-type character) were dissolved in water, mixed, heated to dryness, and decomposed at 250°-300°C. The oxide mixtures were then pelleted and sintered 4 hr at 800° in order to establish the disorder equilibrium of the doped zinc oxide. The ratio Ni/ZnO was 1 8 and the eventual doping amounted to 0.2 at % (75). 

A typical example of the application of EIS is the investigation of passive films on Zn, Zn-Co, and Zn-Ni (Fig. 7-18), which were carried out to explain the difference in the corrosion behavior of pure and low-alloyed zinc by the possible formation of electron traps through the incorporation of cobalt or nickel into the oxide film (Vilche et al., 1989). Passive films of zinc in alkaline solutions are known to be n-type semiconductors with a band gap Eg = 3.2 eV (Vilche et al., 1989). The n-type character arises from an excess of zinc atoms in the nonstoichiometric oxide. The impedance measurements in 1 N NaOH solution were carried out at potentials at which Faraday reactions like transpassive dissolution and oxygen evolution do not interfere. The passive layer was formed for 2 h at positive potential before the potential was swept in the negative direction for the impedance meas-... 

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