**1. Introduction**

78 Artificial Photosynthesis

Zouni, A., Witt, H., Kern, J., Fromme, P., Krauss, N., Saenger, W. & Orth., P. (2001) Crystal

Vol. 409, pp. 739-743.

structure of photosystem II from *Synechococcus elongatus* at 3.8 Å resolution. *Nature*,

It is commonly accepted that "Organic Synthesis" was born in 1828, when the german chemist Friedrich Wöhler succeeded to make urea from simple materials (Friedrich, 1828). After only few years, Becquerel reported the first example of photo-induced electrochemical reaction (Becquerel 1839). In his work, he found that a voltage and an electric current were obtained by illuminating a silver chloride electrode immersed in an electrolyte solution and connected to a counter electrode. But the world had to wait until 1955, when Brattain and Garret, working on germanium semiconductor electrodes, well understood the origin of that strange photovoltaic phenomenon (Brattain & Garret, 1955). The modern "Photoelectrochemistry" era was born.

However, the year was 1900 when it appeared the first article onto the use of light to promote an organic reaction (Albini & Fagnoni, 2008; Ciamician & Silber 1900). Ciamician and his colleague Silber carried out the first systematic studies on the behaviour of organic substances in the presence of sunlight, thus, nowadays they are regarded as the fathers of modern Organic Photochemistry.

Nowadays, if we open SciFinder Scholar client and we write the word "Synthesis" in the "Research topic" field, almost 10 millions of references will be shown. This means that in the last two centuries more than 50000 papers per year onto the argument have been published. Moreover, we are considering only an average value, whereas it is well known that the number of publications has known an exponential-type growth in the last decades. However, only 70000 references are reported for "Photocatalysis", indicating how relatively young is this research field, although the first paper was published more than a century ago.

The question is: why in the last two-three decades the use of light in chemistry has aroused a such worldwide attention? The answer could be summarized mainly in one word: environment.

A great impetus to the development of photocatalysis is derived from the growing demand to reduce the environmental pollution (air and ground). This has led to the development of several photo-induced protocols for the oxidative degradation of organic pollutants, in which semiconductors play a key role in the reaction mechanism, focusing the attention onto the photodegradation of water and air pollutants.

Semiconductors in Organic Photosynthesis 81

consider EF as the energy for which the occupancy probability is 0.5, EF must move closer to the CB for a *n*-type semiconductor (Fig. 1 scheme on the right). Similarly, EF must move closer to the VB for a *p*-type semiconductor (Fig. 1 scheme on the left). In the extreme cases, in which the doping level is very high, the Fermi levels move into the CB for *n*-type

Fig. 1. Energy diagram and Fermi distribution function *f(E)* for semiconductors

The application of several *n*-type semiconductors on photocatalysis has experienced an exponential growth in the last 20 years (Fox & Dulay, 1993; Hashimoto et al., 2005; Hoffmann et al., 1995; Maldotti et al., 2002a). Let we consider now the influence of photo-

When a semiconductor surface is brought into contact with an electrolyte containing a redox couple, interfacial electron transfer can transpire, to equilibrate the solution phase potential and the energy levels of the bulk semiconductor. As electron exchange takes place across the interface, equilibration with the solution occurs and the bulk EF moves to the solution-phase equilibrium potential, whereas the band-edge positions at the surface remain fixed at their original values (Fig. 2). Thus, in moving inward from the semiconductor-electrolyte

Fig. 2. Band bending into *n*-type and *p*-type semiconductors. The vertical lines represent

**2.2 Electrical properties of illuminated** *n***-type semiconductors** 

irradiation onto *n*-type semiconductor properties (Fox, 2001).

semiconductor / solution interface.

semiconductor and into the VB for *p*-type semiconductor.
