Z-scheme

The Z Scheme of Photo.syndiedc Electron Tran.sfer... 

A cyclic conjugate molecule composed of n bonds, A, B,..., X, and Y interacts at A with a reactant Z (Scheme 12). When the molecule is an electron donor (Scheme 12a), electrons delocalize from a to z. The resulting electron hole in a is supplied with an electron by the neighboring b. Similar delocalization sequentially follows from c to b, from d to c and so on. This is also the case with the opposite side Y, X,. It follows that the cyclic orbital interaction of a, b,.x, andy is important in the conjugated molecule. The orbitals are all electron-donating orbitals. When each neighboring pair of orbitals is combined out of phase, the interaction of the cyclic... 

Figure 12.2a. Photosynthetic Z-scheme for green plants. Abbreviations not included in the text are PQ, plastiquinone Cyt bse, a form of cytochrome b absorbing at 564 nm FD, ferredoxin FP a flavoprotein. Long vertical arrows indicate steps arising from photoactivation of pigment reaction centers dashed arrows indicate uncertain pathways.0185...

Fig. 5.8. When photosystem II is activated by absorbing photons, electrons are passed along an electron-acceptor chain and are eventually donated to photosystem I and finally to NAPD+. Photosystem II is responsible for the photolytic dissociation of water and the production of atmospheric oxygen. This pathway is sometimes referred to as the Z scheme because of its zigzag route, as depicted here, but the two arms are in fact remote in space. (Note Plastocyanin (Cu) is an alternative late replacement for an Fe cytochrome complex).

Figure 22 The energy level diagram (Z-scheme) for photocatalytic water decomposition by a tandem cell.

Integration of PS II and PS I via cyt b6f in chloroplasts (Z scheme of photosynthesis). Pheo a represents pheophytin pQA and pQB represent phytoquinone A and B, respectively A, and At represent electron acceptor chlorophylls, respectively pc represents plastocyanin and Fd represents ferridoxin. 

The pathway of electron flow is called the Z scheme of photosynthesis because the diagram from p680 to p700 resembles the letter Z (Figure 7.10) [37,39]. 

Linkous, C.A. and Slattery, D.K., Solar hydrogen via photosynthetic Z-scheme analogue based on semiconducting powders, Am. Chem. Soc. Fuel Chem., 43, 378, 1998. 

Of the photocatalytic systems and structures composed of a single active material, eventually coupled with redox catalysts and/or metals, only a wide band gap oxide semiconductor, like Pt/Ti02, requiring UV irradiation, showed some photoactivity for water photosplitting. Water splitting with visible light requires the irradiation of multiple band gap photoelectrochemical cells (PEC) or Z-scheme systems (like the photosynthesis system of plants etc.). 

H. Kato, M. Hori, R. Konta, Y. Shimodaira, A. Kudo, Construction of Z-scheme type heterogeneous photocatalysis systems for water splitting into H2 and 02 under visible light irradiation, Chem. Lett. 33 (2004) 1348-1349. 

This system bears similarity to the so-called Z scheme for photosynthesis (see Chapter 12). 

Figure 12.8 The Z scheme, an overview of the flow of electrons during the light-dependent reactions of photosynthesis. ED and EA refer to the electron donors and acceptors of the two photosystems, respectively...

The water acts as a large reserve of electrons for the photosynthetic electron transport process in the Z scheme, with the oxygen being produced as a waste product. 

Carbon nanodots and other carbon surface species may act as efficient solid-state sensitizers to promote visible light absorption. In addition, various nanocarbons may act as semiconductors enabling to realize interesting Z-schemes for extended visible light activity. 

Qian, Z. Pathak, B. Nisar,). Ahuja, R., Oxygen- and nitrogen-chemisorbed carbon nanostructures for Z-scheme photocatalysis applications. /. Nanoparticle Res. 2012,14 895/1-895/7. 

Iwase, A. Ng, Y. H. Ishiguro, Y. Kudo, A. Amal, R., Reduced Graphene Oxide as a Solid-State Electron Mediator in Z-Scheme Photocatalytic Water Splitting under Visible Light./. 

Z-scheme of oxygenic photosynthesis in green algae cyanobacteria, showing links to hydrogenase... 

Figure 10.3 Z-scheme of oxygenic photosynthesis in green algae and cyanobacteria, showing links to hydrogenase. Q (plastoquinone) and X (an iron-sulfur cluster) are electron acceptors from photosystems II and I, respectively.The two hydrogenases shown are the NADP-dependent bidirectional hydrogenase and a ferredoxin-dependent enzyme.

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