Photoelectrochemical deposition of metals

The image is usually formed via photoelectrochemical deposition of metals (Pd, Ag, etc.). Metallic particles either directly form the image of a sufficient optical density (Goryachev et al, 1970, 1972) or serve as nuclei of crystallization in the course of development (Kelly and Vondeling, 1975). Photoelectrochemical reactions of colored organic compounds may also be employed for producing image (see, for example, Reichman et al, 1980). 

Photocatalytic and photoelectrochemical deposition of metals onto semiconductor surfaces has been of considerable interest for improving the characteristics of both the... 

Mechanisms of electrochemical and photoelectrochemical deposition of metal selenide clusters (Me = Pb, Cd, Zn, Bi, In) onto the surface as well as into the selenium films have been studied. These clusters are formed as a result of underpotential and overpotential deposition of the metals onto Se. Photoinduced underpotential deposition of Bi onto Se was used to cover selenium colloidal particles with BiiSes clusters. The PbSe and Bi2Sc3 clusters modify the Se surface and form electronic surface states in the Se bandgap, thus promoting electron exchange processes between the valence band and redox species in solution and the rise of the subbandgap photocurrent. 

Rose, T.L., Longendorfer, D.H. and Rauh, R.D., Photoelectrochemical deposition of metals onto p-silicon using an internal ceil. Appl. Phys.Lett. 42 193-195(1983)... 

Photoelectrochemical behavior of metal phthalocyanine solid films (p-type photoconductors) have been studied at both metal (93,94,95,96) and semiconductor (97,98) electrodes. Copper phthalocyanine vacuum-deposited on a Sn02 OTE (97) displayed photocurrents with signs depending on the thickness of film as well as the electrode potential. Besides anodic photocurrents due to normal dye sensitization phenomenon on an n-type semiconductor, enhanced cathodic photocurrents were observed with thicker films due to a bulk effect (p-type photoconductivity) of the dye layer. Meier et al. (9j>) studied the cathodic photocurrent behavior of various metal phthalocyanines on platinum electrodes where the dye layer acted as a typical p-type organic semiconductor. 

It is worth noting that in the processes of the direct photochemical or photoelectrochemical synthesis of Bi nanophase being considered, under the conditions of profound photolysis (at the pronounced darkening), the Bi particles range up to 2-5 nm according to the data of transmission electron microscopy. It is clear that under less exposure Bi particles may have essentially less size (the circumstantial evidence is that such particles possess high chemical reactivity and are very unstable on exposure to the air). It is possible to visualize them by the deposition of other metals (e.g. Ag) from the especial solutions employed for the autocatalytic chemical deposition of metals (so called physical developers ) [91]. 

Electrocatalytic, photocatalytic, and photoelectrochemical behavior of a semiconductor modified by the deposition of metal nanoparticles depends strongly on the... 

Michaels, R. H., A. D. Darrow II, and R. D. Rauh. Photoelectrochemical deposition of microscopic metal film patterns on Si and GaAs. 

As with etching, it is also possible in some cases to observe photoelectrochemical deposition of thin films without external control, i.e., an electroless process (31)(38). In this case, the semiconductor is simply Immersed in the piating/precursor solution and the surface illuminated with the desired pattern, The main requirement for the electroless photodeposition of a metal to occur is to have a sacrificial reductant in solution or on the semiconductor (e.g., the semiconductor itself). Thus, when the semiconductor Is illuminated with the light pattern, the metal lonswillbereduced in the Illuminated zones and the reductant oxidized by majority carriers in the dark zones. 

Boschloo GK, Goossens A, Schoonman J (1997) J Photoelectrochemical study of thin film of anatase Ti02 films prepared by metal organic chemical vapor deposition. J Electrochem Soc 144 1311-1317... 

This description of the photoelectrochemical event was originally based on the use of an illuminated semiconductor electrode in a standard three electrode cell configuration. The theory can easily be extended, however, for practical applications to short-circuited cells prepared by deposition of inert metal with low overvoltage characteristics on a powdered semiconductor Such a metallized powder is shown in... 

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. 

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. 

The surface concentration, size distribution and other properties of metal nanoparticles formed in a dark on the surface of the inert wide-band-gap semiconducting oxides under contact, photocatalytic, or photoelectrochemical deposition depend substantially on the concentration, bulk distribution, and energy characteristics of donor defects in the initial semiconductor substrate. As a rule, the necessary condition for the formation of the smallest-sized particles in the highest surface concentration is the maximum shift of the surface potential of semiconducting matrix from its equilibrium value during metal deposition. This is part of the reason for the experimentally observed fact that the particles formed in the condition of photocatalytic deposition are characterized by less average size and cover superior portion of surface than those obtained under cathodic deposition, all other factors being equal. 

If one conceptually considers making the particle smaller and smaller, one eventually reaches a colloidal state. Colloidal forms of metal oxide semiconductors can often be prepared by hydrolysis of an appropriate organometallic precursor, and the resulting suspension may represent the ultimate miniature photoelectrochemical cell [28]. The resulting colloid can then be used as synthesized or it can be deposited as a thin film on an inert support. 

Noble metal particles of Au, Pt, and Ir were deposited on nanostructured Ti02 films using an electrophoretic approach [189], The improved photoelectrochemical performance of the semiconductor-metal composite film was attributed to the shift in the quasi-Fermi level of the composite to more negative potentials. Continuous irradiation of the composite films over a long period causes the photocurrent to decrease as the semiconductor-metal interface undergoes chemical changes. 

Chlorophyll-containing Membrane on an Electrode Surface. Photoelectrochemical and photovoltaic effects have been noted with a variety of chlorophyll assemblies deposited on metal or semiconductor electrode surfaces (Figure 6). 

Around 1975, investigations of photoelectrochemical reactions at semiconductor electrodes were begun in many research groups, with respect to their application in solar energy conversion systems (for details see Chapter 11). In this context, various scientists have also studied the problem of catalysing redox reactions, for instance, in order to reduce surface recombination and corrosion processes. Mostly noble metals, such as Pt, Pd, Ru and Rh, or metal oxides (RUO2) have been deposited as possible catalysts on the semiconductor surface. This technique has been particularly applied in the case of suspensions or colloidal solutions of semiconductor particles [101]. However, it is rather difficult to prove a real catalytic property, because a deposition of a metal layer leads usually to the formation of a rectifying Schottky junction at the metal-semiconductor interface (compare with Chapter 2), as will be discussed below in more... 

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