Photoelectrochemical metal electrodes

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. 

In their pioneering work on the formation of photoelectrochemically active metal sulfides by oxidation of the parent metal electrode. Miller and Heller [29] reported the anodic formation of polycrystalline Bi2S3 on a bismuth metal electrode in a sodium polysulfide cell, wherein this electrode was used in situ as photoanode. When a Bi metal electrode is anodized in aqueous sulfide solutions a surface film is formed by the reaction... 

The first observations of photoelectrochemical phenomena were made in 1839 by Antoine Becquerel (1788-1879). He used symmetric galvanic cells consisting of two identical metal electrodes in a dilute acid. When illuminating one of the electrodes he observed current flow in the closed electric circuit. 

Semiconductor electrodes exhibit electron photoemission into the solution, like metal electrodes, but in addition they exhibit further photoelectrochemical effects due to excitation of the electrode under illumination. The first observations in this area were made toward the middle of the twentieth century. At the end of the 1940s,... 

Prior to the 1970 s, electrochemical kinetic studies were largely directed towards faradaic reactions occurring at metal electrodes. While certain questions remain unanswered, a combination of theoretical and experimental studies has produced a relatively mature picture of electron transfer at the metal-solution interface f1-41. Recent interest in photoelectrochemical processes has extended the interest in electrochemical kinetics to semiconductor electrodes f5-151. Despite the pioneering work of Gerischer (11-141 and Memming (15), many aspects of electron transfer kinetics at the semiconductor-solution interface remain controversial or unexplained. 

At present there is a sufficiently complete picture of photoelectrochemical behavior of the most important semiconductor materials. This is not, however, the only merit of photoelectrochemistry of semiconductors. First, photoelectrochemistry of semiconductors has stimulated the study of photoprocesses on materials, which are not conventional for electrochemistry, namely on insulators (Mehl and Hale, 1967 Gerischer and Willig, 1976). The basic concepts and mathematical formalism of electrochemistry and photoelectrochemistry of semiconductors have successfully been used in this study. Second, photoelectrochemistry of semiconductors has provided possibilities, unique in certain cases, of studying thermodynamic and kinetic characteristics of photoexcited particles in the solution and electrode, and also processes of electron transfer with these particles involved. (Note that the processes of quenching of photoexcited reactants often prevent from the performing of such investigations on metal electrodes.) The study of photo-electrochemical processes under the excitation of the electron-hole ensemble of a semiconductor permits the direct experimental verification of the applicability of the Fermi quasilevel concept to the description of electron transitions at an interface. 

In fact, the surface may mediate the requisite chemistry of the initially formed radical cation so that different products can be observed from the same intermediate when generated photoelectrochemically or by other means. The radical cation of diphenyl-ethylene, for example, gives completely different products upon photoelectrochemical activation 2 than upon electrochemical oxidation at a metal electrode or by single electron transfer in homogeneous solution, Eq. (31) . Surface control of... 

While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

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. 

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... 

Chlorophyll-Coated Metal Electrodes. Photoelectrochemical reactions at Chl-coated metal electrodes have been investigated respecting the various configurational modes of Chi. Platinum electrodes have widely been employed as substrates of Chi films. 

A significant drawback of metals for photoelectrochemical applications lies in their ability to efficiently quench excited states via energy transfer processes, as discussed below. Direct detection of photosensitized electron transfer to or from a metal electrode surface has been observed [30]. However, unlike dye-sensitized semiconductor systems, little examination of the kinetics of such systems has yet been undertaken. 

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. 

Before considering the kinetics of photoelectrochemical reactions, it is useful to compare electron transfer at metal electrodes with electron trans-... 

A p-type silicon (p-Si) electrode modified with copper particles (particulate-Cu/p-Si) was applied to photoelectrochemical (PEC) reduction of carbon dioxide (CO2) in acetonitrile electrolyte solutions with and without 3.0 M HjO. The particulate-Cu/p-Si electrode generated high photovoltages of 0.50 to 0.75 V, and produced methane, ethylene, etc., under addition of 3.0 M HjO, similar to a Cu metal electrode, indicating that the particulate-Cu/p-Si electrode acted as an efficient electrode for the PEC reduction of CO2 in non-aqueous solutions. 

Single crystal p-Si(lOO) wafers [CZ, 1.37 - 1.7 I2cm, Shin-Etsu Handotai Co., Ltd.] were used. Ohmic contact was made with In-Ga-Zn alloy. Metal (Cu) particles were deposited photoelectrochemically in a 0.01 M CUSO4 acidic solution. Details of the electrode preparation were described in our previous papers [2-4], Copper metal electrodes were prepared using a Cu sheet (99.994 %). They were electrochemically polished at 2.0 V vs. Cu plate counter-electrode in 85.0 % phosphoric acid for ca. 1 min. 

Light energy may be used to reduce the necessary electrical potential in photoelectrochemical reactions. The overpotential is decreased by 700 mV for the photoelectrochemical reduction of CO on p-CdTe, compared to that on indium - the best metal electrode for CO2 reduction. For these semiconductors which involve a high concentration of surface states, the double layer at the semiconductor-electrolyte interface plays an important role in the kinetics of photoelectrochemical reactions. In this paper, we report spectroscopic and impedance aspects of the electrode-electrolyte interface as affected by reactants and radicals involved in CO reduction. 

This section presents results that show how the rates of photoelectrochemical processes can be derived from time resolved measurement of the photoinduced current or potential in the external circuit of a photoelectrochemical cell. The capacitance of the Helmholtz-double layer is of the order of lO Fcm , the depletion layer capacitance of an extrinsic semiconductor junction is typically 10 -10 Fcm , while the capacitance of an insulator is orders of magnitude lower. With a value of 100 Ohm for the resistance Rd + R of the cell, the time constant of photoelectrochemical cells is 10 s for metallic electrodes, 10 -10" s for semiconductor electrodes and much lower for insulator electrodes. The rates of photoelectrochemical processes also span a wide range. This makes photoelectrochemical kinetics a rich, albeit demanding, area for research. 

Short wavelength photons (ofenergymuch greater than Eg) create hot carriers. If, somehow, thermalization of these carriers can be avoided, photoelectrochemical reactions that would otherwise be impossible with the cooled counterparts, that is, at very negative potentials for n-type semiconductors, would be an intriguing possibility. The key issue here is whether the rate of electron transfer across the interface can exceed the rate of hot electron cooling. The observation of hot carrier effects at semiconductor-electrolyte interfaces is a controversial matter [3,7,11,171] and practical difficulties include problems with band edge movement at the interface and the like [4]. Under certain circumstances (e.g. quantum-well electrodes, oxide film-covered metallic electrodes), it has been claimed that hot carrier transfer can indeed be sustained across the semiconductor-electrolyte interface [7,172,173]. 

Consider a metal working electrode first. The ejected electrons from a metal electrode surface will travel a few A into the electrolyte phase and then become solvated. These solvated species display interesting chemistry and electrochemistry if suitable electron scavengers are available to interact with them. The resultant free radical species can be probed spectroscopically in these photoelectrochemical or spectrophotoelectro-chemical experiments. The irradiation can be either continuous or pulsed and with the advent of powerful laser sources a whole slew of experimental strategies open up [9,10]. Of course with continuous irradiation the spectroscopic probe will have to be orthogonally placed to avoid interference with the incoming radiation [11]. 

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