Photocatalytic Activation

As a result of solid-state quantum effects, semiconductor materials possess two allowable electron energy bands. The lower energy region is the valence band; electrons in this energy band are binding electrons and are somewhat restricted in movement. The higher region is the conduction band. These electrons, to a first approximation, are free to move throughout the solid and produce conductivity similar to that of metals. Between these two regions is a forbidden zone, or bandgap.

Photoexcitation in a semiconductor occurs as the absorption of radiation of energy equal to, or greater than, the bandgap energy excites an electron (e~) into the conduction band of the solid. There is, correspondingly, an electron vacancy or hole (h+) that remains in the valence band (Figure 1).

Potential (V)

Reduced Product

Oxidized Product

Reduced Reactant

Oxidized Reactant

Figure 1 Creation of electron-hole pairs in illuminated semiconductors, and subsequent photocatalytic redox reactions.

Reduced Product

Oxidized Product

Reduced Reactant

Oxidized Reactant

Figure 1 Creation of electron-hole pairs in illuminated semiconductors, and subsequent photocatalytic redox reactions.

Table 2 Some Common Semiconductors, Their Bandgap Energy (pH 0), and Corresponding Excitation Wavelength

Semiconductor

Bandgap (eV)

Wavelength (nm)

Ti02

3.0-3.2

413-388

ZnO

3.2

388

ZnS

3.7

335

CdS

2.4

516

Fe203

2.3

539

wo3

2.8

443

Source: Maruska and Ghosh [4]; Sakata and Kawai [5]

Source: Maruska and Ghosh [4]; Sakata and Kawai [5]

These holes, having an affinity for electrons, are very strong oxidizing agents. The number of electron-hole pairs is dependent on the intensity of the incident light and the material's electronic characteristics that prevent them from recombining and releasing the absorbed energy. The electron is free to move throughout the solid in the nearly unoccupied conduction band. Similarly, the hole can migrate by a valence band electron filling the vacancy, leaving behind another hole in the previous position. The bandgap energy and corresponding wavelength required for excitation for some common semiconductors are given in Table 2.

The semiconductor potentials for the valence band and conduction band are significantly different. This difference avoids rapid recombination of the e~-h + pairs. The band potentials are a function of pH and decrease by 0.059 V per pH unit increase as predicted by the Nernst equation [6,7],

The holes in the semiconductor solid are attracted to the oxide/sulfide surface, where they oxidize an adsorbed water molecule or hydroxide ion.

Hydroxy 1 radicals are very reactive neutral species with an unpaired electron. They react rapidly and nonselectively in the oxidation of organic compounds and are the common oxidizers in AOP and high-pH ozone systems.

Reaction (2) is likely to occur for two reasons. One is that large quantities of OH- and HzO groups are available as adsorbates, and the chances of holes reacting with these groups on the semiconductor surface are high. The second reason is that for several semiconductors the oxidation potentials of these reactions are above (more negative than) the potential for the valance band over the entire pH range. Reaction (2a) is favored at low pH and reaction (2b) at higher pH values [7].

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