**4. Water oxidizing complex**

The hydrogen production from water splitting is an appealing solution for the future energy as discussed by Bockris (Bockris, 1977). A strategy is to employ solar, wind, ocean currents, tides or waves energy to water splitting. However, to evolve hydrogen efficiently in a sustainable manner, it is necessary first to synthesize a stable, low cost, and efficient, environmentally friendly and easy to use, synthesis, manufacture catalyst for water oxidation, which is the more challenging half reaction of water splitting. In past few years, there has been a tremendous surge in research on the synthesis of various metal compounds aimed at simulating water oxidizing complex (WOC) of photosystem II. Particular attention has been given to the manganese compounds aimed at simulating the water oxidizing complex of photosystem II (Umena et al., 2011)) not only because it has been used by *Nature* to oxidize water but also because manganese is cheap and environmentally friendly.

### **5. The dye-sensitized solar cell approach**

Photovoltaic is a method of generating electricity by converting light into electricity. Photovoltaic devices work base on the concept of charge separation. A new family of devices, the dye-sensitzed solar cell, is shown in Fig. 2. In the system, there is an oxide layer (for example TiO2) which to allow for electronic conduction to take place. The material oxide layer is attached to a monolayer of the charge transfer dye. Photo excitation of the dye results in the injection of an electron into the conduction band of the oxide. Usually the iodide/triiodide couple restors the original state of the dye. The dye-sensitized solar cell made of low-cost materials, robust, does not require elaborate apparatus to manufacture, can be engineered into flexible sheets, requiring no protection from minor events like hail or tree strikes. Thus, there are technically attractive. In this devise, light is absorbed by a dye, the sensitizer is grafted onto the TiO2 surface and then light induced electron injection from the adsorbed dye into the TiO2 conductive (Grätzel, 2003).

In Fig. 3 comparing between the spectral response of the photocurrent observed with the two sensitizers and TiO2 is shown (Grätzel, 2003). The incident photon to current conversion efficiency of the dye-sensitized solar cell is plotted as a function of excitation wavelength. Both chromophores show very high incident photon to current conversion efficiency values in the visible range. Some companies work to develop dye-sensitized solar cells for applications in cars and homes.

Gathering Light: Artificial Photosynthesis 7

Fig. 3. The incident photon to current conversion efficiency of the dye-sensitized solar cell is plotted as a function of excitation wavelength. (The figure was reproduced from (Grätzel,

2003)).

Fig. 2. A schematic presentation of the operating principles of the dye-sensitized solar cell (The figure was reproduced from Grätzel, 2003).

Fig. 2. A schematic presentation of the operating principles of the dye-sensitized solar cell

(The figure was reproduced from Grätzel, 2003).

Fig. 3. The incident photon to current conversion efficiency of the dye-sensitized solar cell is plotted as a function of excitation wavelength. (The figure was reproduced from (Grätzel, 2003)).

Gathering Light: Artificial Photosynthesis 9

form of energy-rich molecules such as glucose. RuBisCO catalyzes either the carboxylation

It is believed that RuBisCO is rate-limiting for photosynthesis in plants and it is proposed that may be possible to improve photosynthetic efficiency by modifying RuBisCO genes in plants to increase its catalytic activity (Spreitzer & Salvucci, 2002). Engineered changes in Rubisco's properties Unpredictable expression of plastid transgenes and assembly requirements of some foreign Rubiscos that are not satisfied in higher-plant plastids provide

There are the most important titles in artificial photosynthesis but we could increase our list as important titles in artificial photosynthesis and as it is considered by Pace, artificial photosynthesis is an umbrella term. As you see in each title, inspired by natural photosynthesis, in artificial photosynthesis novel approaches used to develop technologies for non-polluting electricity generation, fuel production and carbon sequestration using solar energy. (Pace, 2005). Researchers and scientists are trying to learn a great about the detail of natural photosynthetic systems and have been able to understand at least parts of this process. Therefore, artificial photosynthetic goal and capable of converting sunlight into chemically-bound energy seem to be a realistic

Authors are grateful to Institute for Advanced Studies in Basic Sciences for financial

Govindjee; Kern, J.F.; Messinger, J.& Whitmarsh, J. (2010) Photosystem II. In: Encyclopedia

Grätzel, M. (2003) Dye-sensitized solar cells. *J. Photochem. Photobiol., C*, vol.4, pp. 145-

Moore, T. A. (2005) Bio-inspired energy security for planet earth. Photochem. Photobiol. Sci,

Pace, 2005 R. J.: An Integrated Artificial Photosynthesis Model. In: Collings, A. F. &

Rossi, F. & Filipponi, M. (2011) Hydrogen production from biological systems under different illumination conditions. *Int. J. Hydrogen. Energ*., v.36, p7479-7486. Sakata, Y.; Imahori, H. & Sugiura, K. (2001) Molecule-based artificial photosynthesis. *J. Incl.* 

Spreitzer, R.J. & Salvucci, M.E. (2002). Rubisco: structure, regulatory interactions, and possibilities for a better enzyme. *Annu. Rev. Plant. Biol*., v.53, pp. 449–475.

Critchley, C. (eds.): Artificial Photosynthesis: From Basic Biology to Industrial

In: Collings, A. F. & Critchley, C. (eds.): Artificial Photosynthesis: From Basic

Bockris, J.O.M. (1977) Energy-the solar hydrogen alternative, Wiley&Sons, New York. Cook; P. J. (2005). Greenhouse gas technologies: A pathway to decreasing carbon intensity.

Biology to Industrial Application. Weinheim 2005, pp. 13-34).

of Life Sciences (ELS). John Wiley & Sons, Ltd: Chichester

or the oxygenation of ribulose-1,5-bisphosphate.

challenges for future research.

scenario in near future.

**9. Acknowledgment** 

**10. References** 

153.

v.4, pp. 927-927).

Application. Weinheim 2005, pp. 13-34.

*Phenom. Macro*., v.41, 31-36.

support.
