Semiconducting minerals

In addition to photocleavage of water with photocatalysts, other photosynthetic processes such as photochemical C02 reduction, resulting in the formation of CO or methane, and the photochemical fixation of nitrogen are of great interest  

The electronic properties of solids can be described by various theories which complement each other. For example band theory is suited for the analysis of the effect of a crystal lattice on the energy of the electrons. When the isolated atoms, which are characterized by filled or vacant orbitals, are assembled into a lattice containing ca. 5 x 1022 atoms cm 3, new molecular orbitals form (Bard, 1980). These orbitals are so closely spaced that they form essentially continuous bands the filled bonding orbitals form the valence band (vb) and the vacant antibonding orbitals form the conduction band (cb) (Fig. 10.5). These bands are separated by a forbidden region or band gap of energy Eg (eV). 

An Intrinsic Semiconductor is characterized by an equal density of positive and negative charge carriers, produced by thermal excitation i.e., the density of electrons in the conduction band, nj, and of holes in the valence band, Pi, are equal 

Formation of bands in solids by assembly of isolated atoms into a lattice (modified from Bard, 1980). When the band gap Eg kT or when the conduction and valence band overlap, the material is a good conductor of electricity (metals). Under these circumstances, there exist in the solid filled and vacant electronic energy levels at virtually the same energy, so that an electron can move from one level to another with only a small energy of activation. For larger values of Eg, thermal excitation or excitation by absorption of light may transfer an electron from the valence band to the conduction band. There the electron is capable of moving freely to vacant levels. The electron in the conduction band leaves behind a hole in the valence band. 

Extrinsic Semiconductors are materials that contain donor or acceptor species (called doping substances) that provide electrons to the conduction band or holes to the valence band. If donor impurities (donating electrons) are present in minerals, the conduction is mainly by way of electrons, and the material is called an n-type semiconductor. If acceptors are the major impurities present, conduction is mainly by way of holes and the material is called a p-type semiconductor. For instance in a silicon semiconductor elements from a vertical row to the right of Si of 

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Of major interest in geochemistry and in natural water systems are semiconducting minerals for which the absorption of light occurs in the near UV or visible spectral region and as a result of which redox processes at the mineral-water interface are induced or enhanced. Table 10.1 gives band gap energies of a variety of semiconductors. 

In heterogeneous photoredox reactions not only the solid phase, i.e. the semiconducting mineral, may act as the chromophore (as discussed in Chapter 10.2) but also a surface species (i) a surface complex formed from a surface metal ion of a metal (hydr)oxide and a ligand that is specifically adsorbed at the surface of the solid phase, and (ii) a chromophore that is specifically adsorbed at the surface of a solid phase. In the following these three cases will briefly be discussed. 

Current research is showing the interactions between organic molecules and a wide array of minerals. These include the formation of carboxylic acids in thermal vent chemistry and the formation of reduced chemical species through photochemistry involving semiconducting minerals.8... 

Table 2 gives electrical, magnetic, and optical properties, while Tables 3 and 4 give more details on the semiconducting properties and band structures of the most common semiconductors. Table 5 lists semiconducting minerals with typical resistivity ranges. 

Radiative Transition Probabilities for X-Ray Lines Radioactive Tracer Diffusion Data for Pure Metals Recommended Daily Dietary Allowances Reduction of Barometer to Sea Level Refractory Materials Resistance of Wires Resistivity of Semiconducting Minerals Solvents for Liquid Chromatography Spark-Gap Voltages... 

Xu Y, Schoonen MAA The absolute energy positions of conduction and valence bands of selected semiconducting minerals. American Mineralogist 2000, 85(3-4) 543-556. 

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