Metal photoelectrochemical systems

Whether or not Chi is regarded intrinsically as an organic semiconductor, the solid Chi layer in contact with a metal does display a p-type photovoltaic effect, and its efficiency depends significantly on the morphology of the Chi layer as well as the nature of the metal. The effect corresponding to a p-type photoconductor can also be expected at the junction of a metal / Chi / liquid in a photoelectrochemical system. Such a presumption is in fact compatible with the photoelectrochemical behavior observed for most of Chl-coated metal electrodes, as will be shown later. 

Photoelectrochemical Systems Involving Chlorophyll-Coated Semiconductor and Metal Electrodes... 

Quantum chemistry has so far had little impact on the field of photoelectrochemistry. This is largely due to the molecular complexity of the experimental systems, which has prevented reliable computational methods to be used on realistic model systems, although some theoretical approaches to various aspects of the performance of nanostructured metal oxide photoelectrochemical systems have appeared in the last 10 years, see e.g. [139, 140, 141]. Here we have focussed on quantum-chemical cluster and surface calculations of a number of relevant problems including adsorbates and intercalation. These calculations illustrate the emerging possibilities of using quantum chemical calculations to model complicated dye-sensitized photoelectrochemical systems. 

There is still a problem in calculation of energy-conversion efficiency when electrochemical or chemical bias is also applied in photoelectrochemical or photocatal5rtic reaction of positive Gibbs energy. For example, as shown in Fig. 5c, energy-conversion efficiency for a photoelectrochemical system consisting of an n-type semiconductor and metal counter electrodes with bias voltage A6 is possibly expressed as follows ... 

Modification of semiconductor electrode response with adsorbed or attached dye molecules is an attractive alternative to other photoelectrochemical systems (7-13). Metal oxides which are stable or have very low corrosion rates but are transparent to visible wavelength light can be used in light-assisted electrochemical reactions when modified with monolayers and multilayers of a wide variety of chromophores interposed between the electrode and electrolyte. With one exception, the initial reports of energy conversion efficiencies of electrodes with adsorbed dyes was disappointingly low. Recently however,... 

The reduction of C02 requires electron transfer in one-electron or multielectron steps either from reducing agents, for example, H2, or electrochemically. H2 can also be produced by water splitting either electrochemically or photochemically. For efficient electrochemical reduction of dissolved CO2, electron transfer catalysts (electron relays, mediators), usually transition metal complexes, are required while photochemical systems need also a photosensitizer. The two approaches can be combined to photoelectrochemical systems, as well. 

Although photoelectrochemical systems are able to offer respectable conversion efficiencies, the refinement of other solution-based processes continues. Gratzel has reviewed photoredox processes, paying particular attention to the use of organized assemblies such as micelles and vesicles. He emphasizes the central role of efficient colloidal metal catalysts in these schemes and also describes the recent development of bifunctional redox catalysts that allow the combination of cycles for the generation of hydrogen and oxygen. 

Fig. 2 Scheme representing the general principle of a photoelectrochemical system. Electrons are photoexdted in the absorber. The electron and hole are selectively transferred to an electron conductor (usually a metal or a semiconductor) and to a hole conductor (a redox system in a liquid electrolyte). The photovoltage is the difference between the electrochemical potentials in the electron and hole conducting phases thus Mh-... 

Most electrochemical and photoelectrochemical systems produce only the two-electron reduction products of CO and formate, the products of two-electron reductions, evidencing once again the kinetic bottleneck associated wth multielectron and multiproton processes. As in the oxidation of water, efforts have been directed towards the development of transition-metal based electrocatalysts with multiple metal centres to facilitate charge accumulation in highly reduced intermediates and allow multiredox processes to occur. " " ... 

On the basis of our theoretical considerations and preliminary experimental work, it is hoped that fast processes of charge carriers will become directly measurable in functioning photoelectrochemical cells, Typical semiconductor electrodes are not the only systems accessible to potential-dependent microwave transient measurements. This technique may also be applied to the interfacial processes of semimetals (metals with energy gaps) or thin oxide or sulfide layers on ordinary metal electrodes. 

Katz E, WiUner 1, Wang J (2004) Electroanalytical and bioelectroanalytical systems based on metal and semiconductor nanoparticles. Electroanalysis 16 19-44 Pardo-Yissar V, Katz E, Wasserman J, Willner 1 (2003) Acetylcholine esterase-labeled CdS nanoparticles on electrodes Photoelectrochemical sensing of the enzyme inhibitors. J Am Chem Soc 125 622-623... 

A number of photochemically or photoelectrochemically activated transition-metal complexes have also been used, both for oxidation and reduction of the nicotinamide cofactors. Among these complexes is the aforementioned Cp Rh(bpy)-complex 9 [52, 53]. For details of these systems or other regeneration procedures using special dyes, the reader is referred to other reviews on coenzyme regeneration [17, 21-23]. 

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.). 

The lure of new physical phenomena and new patterns of chemical reactivity has driven a tremendous surge in the study of nanoscale materials. This activity spans many areas of chemistry. In the specific field of electrochemistry, much of the activity has focused on several areas (a) electrocatalysis with nanoparticles (NPs) of metals supported on various substrates, for example, fuel-cell catalysts comprising Pt or Ag NPs supported on carbon [1,2], (b) the fundamental electrochemical behavior of NPs of noble metals, for example, quantized double-layer charging of thiol-capped Au NPs [3-5], (c) the electrochemical and photoelectrochemical behavior of semiconductor NPs [4, 6-8], and (d) biosensor applications of nanoparticles [9, 10]. These topics have received much attention, and relatively recent reviews of these areas are cited. Considerably less has been reported on the fundamental electrochemical behavior of electroactive NPs that do not fall within these categories. In particular, work is only beginning in the area of the electrochemistry of discrete, electroactive NPs. That is the topic of this review, which discusses the synthesis, interfacial immobilization and electrochemical behavior of electroactive NPs. The review is not intended to be an exhaustive treatment of the area, but rather to give a flavor of the types of systems that have been examined and the types of phenomena that can influence the electrochemical behavior of electroactive NPs. 

Photoelectrochemical conversion from visible light to electric and/or chemical energy using dye-sensitized semiconductor or metal electrodes is a promising system for the in vitro simulation of the plant photosynthetic conversion process, which is considered one of the fundamental subjects of modern and future photoelectrochemistry. Use of chlorophylls(Chls) and related compounds such as porphyrins in photoelectric and photoelectrochemical devices also has been of growing interest because of its close relevance to the photoacts of reaction center Chls in photosynthesis. 

Aizawa and Suzuki (83,84,85,86) utilized, as an ordered system, liquid crystals in which Chi was immobilized. Electrodes were prepared by solvent-evaporating a solution consisting of Chi and a typical nematic liquid crystal, such as n-(p-methoxybenzyl-idene)-p -butylaniline, onto a platinum surface. Chl-liquid crystal electrodes in acidic buffer solutions gave cathodic photocurrents accompanied by the evolution of hydrogen gas (83). This was the first demonstration of photoelectrochemical splitting of water using in vitro Chi. Of particular interest in these studies is the effect of substituting the central metal in the Chi molecule. 

Tetraphenylporphine (TPP) and other metal porphyrine derivatives coated on platinum (87,88,89) or gold (89,90) electrodes have been investigated in photoelectrochemical modes. Photocurrents reported are cathodic or anodic, depending on the pH as well as the composition of the electrolyte employed. Photocurrent quantum efficiencies of 2% (89) to 7% (87) were reported in systems using water itself or methylviologen as the redox species in aqueous electrolyte. Photocurrent generation at Zn-TPP-coated metal cathodes (89) was interpreted in terms of a rectifying effect of the Schottky barrier formed at a metal-p-type... 

The structural studies discussed here deal specifically with the interactions of molecules anchored with carboxylic acids to transition metal oxide surfaces. Bi-isonicotinic acid interacting with metal oxide surfaces combines interest in local carboxylic acid binding, and aromatic interaction with the surfaces. Electronic interactions have been investigated quantum-chemically for several aromatic molecules, including benzoic acid, bi-isonicotinic acid and catechol, all strongly anchored to TiC>2 substrates. Together, these systems represent a significant step towards studies of dye-sensitized metal oxide surfaces in photoelectrochemical devices. 

Large adsorbates, such as bi-isonicotinic acid, may bind to a surface at several sites which are sufficiently far apart not to interact strongly in a direct way. This kind of system is by necessity large and complex, and few detailed studies have been reported on such systems. Various structural aspects of bi-isonicotinic acid adsorption on rutile and anatase TiC>2 surfaces have been presented in several recent studies [68, 77, 78]. Bi-isonicotinic acid adsorption on TiC>2 surfaces is not only taken as a problem of direct interest to the photoelectrochemical applications, but also serves as a model system for surface science investigations of phenomena connected to the adsorption of large organic adsorbates on metal oxide surfaces. 

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