Photocatalyst Photoelectrochemical

Keywords Solar energy Water splitting Hydrogen production Water oxidation Heterogeneous photocatalyst Photoelectrochemical cell... 

The following two papers deal mainly with problems in energy conversion, in piarticular, the transformation of irradiation energy into electrical or chemical energy. The present status and future possible developments of photoelectrochemical energy conversion is presented. In a second paper electrochemical developments are connected to colloidal chemistry and the application of colloidal particles as catalysts for electron transfer reactions and as photocatalysts are discussed. 

Dhere, N.G. and Jahagirdar, A.H., Photoelectrochemical water splitting for hydrogen production using multiple bandgap combination of photovoltaic cell and thin-film-photocatalyst,... 

Despite the relatively large band-gap energy, Ti02 is often used as a photocatalyst because of its relative inertness with regard to dissolution. In order to render such materials more suitable as photocatalysts for the visible part of the solar spectrum, the surface of the solid phase is modified by the specific adsorption of a chromophore which has a lower energy gap between the HOMO and the LUMO as compared to the band-gap of the semiconductor. The principle is shown in Fig. 10.8 for the case of a photoelectrochemical cell. 

Dheere NG, Jahagirdar AH (2005) Photoelectrochemical water splitting for hydrogen production using combination of CIGS2 solar cell and RUO2 photocatalyst. Thin Solid Films 480-481 462-465... 

Applications of titania nanotube arrays have been focused up to now on (i) photoelectrochemical and water photolysis properties, (ii) dye-sensitized solar cells, (iii) photocatalysis, (iv) hydrogen sensing, self-cleaning sensors, and biosensors, (v) materials for photo- and/or electro-chromic effects, and (vi) materials for fabrication of Li-batteries and advanced membranes and/or electrodes for fuel cells. A large part of recent developments in these areas have been discussed in recent reviews.We focus here on the use of these materials as catalysts, even though results are still limited, apart from the use as photocatalysts for which more results are available. 

Even without deposition of a metal island, such powders often maintain photoactivity. The requirement for effective photoelectrochemical conversion on untreated surfaces is that either the oxidation or reduction half reaction occur readily on the dark material upon application of an appropriate potential, so that one of the photogenerated charge carries can be efficiently scavenged. Thus, for some photoinduced redox reactions, metallization of the semiconductor photocatalyst will be essential, whereas for others platinization will have nearly no effect. 

Surface effects and adsorption equilibria thus will significantly influence the course of photoelectrochemical transformations since they will effectively control the movement of reagents from the electrolyte to the photoactivated surface as well as the desorption of products (avoiding overreaction or complete mineralization). The stability and accessibility toward intermolecular reaction of photogenerated intermediates will also be controlled by the photocatalyst surface. Since diffusion and mass transfer to and from the photocatalyst surface will also depend on the solvent and catalyst pretreatment, detailed quantitative descriptions will be difficult to transfer from one experiment to another, although qualitative principles governing these events can be easily recognized. 

Photoelectrochemical hydrogenation of double and triple bonds has been reported with sulfide anion acting as a sacrificial donor, Eq. (35)With CdS metallized with platinum or rhodium as the photocatalyst, hydrogenation was found to be about... 

In Part I the fundamental aspects of photocatalysis are described. Photoelectrochemical processes at semiconductors are the most important basics for all photocatalytic reactions (Chapter 2). Design, preparation and characterization of active photocatalysts have been an important research subject,... 

The early work of photoelectrochemical hydrogen production using Ti02 as catalyst, was reported by Fujishima and Honda [61]. Subsequently, the interest for the photocatalytic processes has grown significantly, although the number of the reported photocatalysts used for water splitting is still limited. 

In spite of a great number of investigations aimed at the preparation of photocatalysts and photoelectrodes based on the semiconductors surface-modified with metal nanoparticles, many factors influencing the photoelectrochemical processes under consideration are not yet clearly understood. Among them are the role of electronic surface (interfacial) states and Schottky barriers at semiconductor / metal nanoparticle interface, the relationship between the efficiency of photoinduced processes and the size of metal particles, the mechanism of the modifying action of such nanoparticles, the influence of the concentration of electronic and other defects in a semiconductor matrix on the peculiarities of metal nanophase formation under different conditions of deposition process (in particular, under different shifts of the electrochemical surface potential from its equilibrium value), etc. 

An example of recent achievement in this area is a flexible, thin film Cu(In,Ga)Se2 solar cell deposited on a titanium foil, which was combined with a TiC>2 photocatalyst layer and modified by a niobium-doped titanium oxide front electrode to function as a photoelectrochemical tandem cell/membrane for a direct light-driven hydrogen evolution from an aqueous solution [48], Under illumination with UV/vis light, the system produced up to 0.052 pLH2/scm2 (e.g. the hydrogen formation rate was approximately 7,250 pmol/h g relative to the amount of TiC>2 used). Several aspects of the operating principles of the photoelectrochemical devices, the materials requirements, main bottlenecks, and the various device concepts (in relation to H2... 

Gracia, F., J.P. Holgado, A. Caballero and A.R. Gonzalez-Elipe (2004). Structural, optical, and photoelectrochemical properties of mn+-Ti02 model thin film photocatalysts. Journal of Physical Chemistry B, 108(45), 17466-17476. 

Table 7 contains a compilation of studies that have appeared since 1985. Several points are worthy of note here. The vast majority of the entries feature studies on Ti02 powders rather than on electrodes in a photoelectrochemical cell configuration. In this light, the new studies can be regarded as offshoots inspired by the earlier (pre-1985) studies on co functional photocatalysts and the cyclic cleavage of water.4 30 Second, many of the new studies address two key issues with the earlier systems ... 

Photocatalysts for phofochemical wafer spliffing can be used for fhis purpose according fo fwo f)rpes of configurafions (i) photoelectrochemical cells and (ii) photocafalyfic sysfems. 

Such particulate systems have the advantage of being much simpler and less expensive to develop and use than photoelectrochemical cells. Furthermore, a wide variety of materials can be used as photocatalysts in... 

When the valence-band hole and the conduction-band electron are both trapped by an appropriate oxidation or reduction half reaction, singly oxidized and reduced species are formed on the surface of the photocatalyst. Because these adsorbed intermediates can move about the surface, either migrating closer together or diffusing away from each other, before back-electron transfer occurs, novel chemistry is likely to ensue. It is a unique characteristic of photoelectrochemical catalysts that both oxidized and reduced species are produced on the same surface. As a result,... 

Photocatalytic activation of allyl and benzyl ethers results either in carbon-carbon coupling or oxygenation [148-151]. The photocatalyst used for these conversions can be generated in situ, by photolysis of a zinc dithiolene salt, by preformed catalysts, or by particles supported within surfactant vesicles. Radical intermediates formed by hydrogen abstraction by photogenerated hydroxyl or hy-droperoxyl radicals may also be important in the photoelectrochemically induced oxidation of hydrocarbons. In the Ti02-sensitized photooxidation of toluene to cresols, for example, a photo-Fenton (radical) type mechanism has been suggested [150]. ... 

---