Absorbed photoelectrochemical

Metal oxide electrodes have been coated with a monolayer of this same diaminosilane (Table 3, No. 5) by contacting the electrodes with a benzene solution of the silane at room temperature (30). Electroactive moieties attached to such silane-treated electrodes undergo electron-transfer reactions with the underlying metal oxide (31). Dye molecules attached to sdylated electrodes absorb light coincident with the absorption spectmm of the dye, which is a first step toward simple production of photoelectrochemical devices (32) (see Photovoltaic cells). 

The overall reaction of the photoelectrochemical cell (PEC), H2O + hv H2 -I- I/2O2, takes place when the energy of the photon absorbed by the photoanode is equal to or larger than the threshold energy of 1.23 eV. At standard conditions water can be reversibly electrolyzed at a potential of 1.23 V, but sustained electrolysis generally requires -1.5 V to overcome the impedance of the PEC. Ideally, a photoelectrochemical cell should operate with no external bias so as to maximize efficiency and ease of construction. When an n-type photoanode is placed in the electrolyte charge distribution occurs, in both the semiconductor and at the semiconductor-... 

Illumination, by light that is absorbed by the growing semiconductor crystals, has been shown to increase the crystal size somewhat, seen as a red shift in the optical spectrum and decrease in bandgap by as much as 0.2 eV [9-11] (see Fig 4.3). This is probably due to photoelectrochemical reactions taking place at the crystal surface. The chemical deposition solution can also be used to electrodeposit CdSe. Electrons (either from an external source, as in the case of elec-... 

Outline of a photoelectrochemical cell based on the reduction of thionine formula given on the right). Light is absorbed by thionine (t) in the illuminated half-cell... 

Applications have been reported for photoelectrochemical experiments, for example, splitting of water [11], local generation of photoelectrodes by spatially selective laser excitation [12], and steady-state electrochemiluminescence at a band electrode array [13,14]. Band electrodes prepared from very thin films approaching molecular dimensions have been used to assess the limits of theory describing electrode kinetics at ultramicroelectrodes [9]. Spectroelectrochemical applications have been extensively reviewed [1], In an intriguing approach, thin, discontinuous metal films have been prepared on a transparent semiconductor substrate they are essentially transparent under conditions in which a continuous metal film containing the same quantity of metal would be expected to substantially absorb [15]. 

The above results indicate that a requirement for water photolysis by Pt/Ti02 is to prevent the reverse reaction on Pt sites. Wagner and Somoijai8) successfully carried out gas-phase water photolysis by Pt/SrTi03-crystal coated with deliquescent basic materials. Their method is reasonable to suppress the reverse reaction, because a deliquescent material coated on a substrate absorbs a large amount of water to form a thin film of its aqueous solution. The film inhibits the reaction products to readsorb directly on the catalyst, while the products on the catalyst can escape to the gas phase by diffusion, it is very important that H2 and 02 can desorb from the catalyst surface to the gas phase without making bubbles, because if they desorb as bubbles then they would inevitably mix with each other in the growing process of bubbles and recombine on Pt sites. In addition, an aqueous basic solution would work as an electrolyte which enhances ion transfer in photoelectrochemical reactions. 

By exciting the red-orange cyclooctatetraene dianion 1 in the presence of cyclooctatetraene in our photoelectrochemical cell (n-TiC>2/NH3/Pt), we were able to observe photocurrents without detectable decomposition of the anionic absorber (2). Presumably, a rapid dismutation of the photooxidized product inhibited electron recombination, producing a stable hydrocarbon whose cathodic reduction at the counter electrode regenerates the original mixture essentially quantitatively (eqn 3). 

Over the past 15 years there has been a wealth of research on development and application of transition metal complex sensitizers to the development of dye sensitized photoelectrochemical (solar) cells (DSSCs) [113]. Charge injection from the excited state of many sensitizers has been found to be on the subpicosecond timescale, and a key objective has been to identify chromophores that absorb throughout the visible spectrum. For this reason, Os(II) complexes appear attractive and a variety of attempts were made to make use of these complexes in DSSCs in the 1990s [114-116]. Work has continued in this area in recent years and representative examples are given below. 

There are three clear divisions in the photoelectrochemical field. In the first, one shines light upon a metal electrode. Here, the theory is well worked out (Barker, 1974 Khan and Uosaki, 1976), but metals absorb light very poorly compared with semiconductors, and this makes the photocurrents obtained by irradiating them extremely small. The second division concerns the absorption of light by molecules in solution and electron transfer from or to these photoactivated species and to or from a conveniently placed electrode (Albery, 1989). Such phenomena are of interest to photochemists, but here the electrode is the handmaiden of the photochemistry and so we regretfully forgo a description of the material. 

The maximum possible energy that a photoelectrode can absorb is all radiation striking it, the photons of which have energy < E if one photoelectrode is used in a photocell and the counter-electrode is a metal. This is, then, the energy available for conversion to new materials by a photoelectrochemical process. 

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