Modified Semiconductor Electrodes

Hinogami et al. deposited small metal particles of Cu, Ag, and Au (20-200 nm diameter) onto p-Si in order to reduce the overpotential for C02 reduction [114], such that the onset potentials were 0.5 V more positive than those of the respective bulk metal electrode. In a C02-saturated aqueous solution, mainly CO was produced with some formic acid however, at —1.05 V (versus SCE) the Ag-coated electrodes produced CO with 51% faradaic efficiency. Similarly, Au-coated electrodes yielded CO with 62% faradaic efficiency at —0.74V (versus SCE). The Cu-coated electrodes, when held at a potential of—1.2 V (versus SCE), produced formic acid and CO with faradaic efficiencies of 32% and 19%, respectively. Small amounts of methane and ethylene were also observed. 

More recently, Kaneco et al. evaluated Pb, Ag, Au, Pd, Cu, and Ni sputter-coated p-InP electrodes [115], Whilst the onset potentials for C02 reduction were less negative than those observed at the corresponding metal electrodes, the authors chose an extremely negative potential of-2.55 V (versus SCE) for their studies. By using a LiOH/methanol electrolyte, they examined the dependence of the amount of metal deposited on the current efficiency of C02 reduction, where the main 

Many of the earliest studies focused on the use of polymer-coated semiconductor materials for the reduction of C02. An example was the study of Aurian-Blajeni et al., who electropolymerized polyaniline onto p-Si [116]. In an aqueous C02-saturated solution, a total faradaic efficiency for formic acid and formaldehyde of 28% was achieved, but at a potential of—1.9 V (versus SCE). Likewise, Cabrera and Abruna electropolymerized [Re(CO)3(v-bpy)Cl], where v-bpy is 4-vinyl-4 -methyl-2,2 -bipyridine [117]. For CO production, TONs of about 450 were observed, while the faradaic efficiencies approached 100%. Upon illumination in acetonitrile solution, the onset potential for reduction was -0.65 V (versus SCE). 

Daube et al. also reported the reduction of C02 at redox polymer-coated p-Si electrode containing a Pd catalyst [118], In an aqueous bicarbonate solution, formic acid was produced with 70% faradaic yield and a small overpotential. 

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This approach can be further extended to photoelectrochemical reactions at modified semiconductor electrodes. In such cases the immobilized substance(s) may serve several functions mediation of the redox process, photosensitization of the semiconductor, and photocorrosion protection. 

Other, less specific potentiometric immunoprobes are based on the antigen-induced potential shift of chemically modified semiconductor electrodes (Fig. 125) (Yamamoto et al., 1983). The surface of a titanium dioxide electrode is covered by a BrCN-activated polymer membrane and inserted into an antibody-containing solution. The antibody binds covalently to the activated electrode surface. The antigen to be determined is added when the potential difference between the sensor and a reference electrode is stable. As a result of the immunological reaction a new... 

Fig. 125. Chemically modified semiconductor electrode with covalently bound antibody for antigen assay. (Redrawn from Yamamoto et al., 1983).

W I Dollard, M L Shumaker, D H Wal-deck, Time-Resolved Studies of Charge Carrier Relaxation in Chemically Modified Semiconductor Electrodes n-CdSe/Silane Interfaces, J. Phys. Chem. 1993, 97,... 

The potential advantages of photoelectrosynthesis over photovol-taics coupled to dark electrosynthesis are in higher net conversion efficiency, better engineering designs for solar reactors, and unique catalytic effects possible with modified semiconductor electrode surfaces. 

Photosensitive substances adsorbed on the semiconductor surface are especially efficient in sensitization reactions. Thus, sensitizing effect can be enhanced if a sensitizer is attached to the semiconductor surface by a chemical bond. For this purpose one has to create either the ether bond -O-between the semiconductor and reactant, using natural OH groups, which exist on the surface of, for example, oxide semiconductors (Ti02, ZnO) or oxidized materials (Ge, GaAs, etc.) in aqueous solutions, or the amide bond -NH- in the latter case a monolayer of silane compounds with amido-groups is preliminarily deposited on the semiconductor surface (see, for instance, Osa and Fujihira, 1976). With such chemically modified electrodes the photocurrent is much higher than with ordinary (naked) semiconductor electrodes. 

Despite extensive studies, the photovoltage or the solar-to-chemical energy conversion efficiency still remains relatively low. The main reason is that it is very difficult to meet all requirements for high efficiency. For example, high catalytic activity and sufficient passivation at the electrode surface are incompatible. It was found, however, that a semiconductor electrode modified with small metal particles can meet all the requirements and thus becomes an ideal type semiconductor electrode. Cu, Ag, and Au were chosen because they were reported to work as efficient electrocatalysts for the C02 reduction. p-Si electrodes modified with these metals in C02-staurated aqueous electrolyte under illumination produce mainly methane and ethylene.178 This is similar to the metal electrodes but the metal-particle-coated electrodes work at approximately 0.5 V more positive potentials, contrary to continuous metal-coated p-Si electrodes. 

The second topic of this chapter is the role of coordination compounds in advancing electrochemical objectives, particularly in the sphere of chemically modified electrodes. This involves the modification of the surface of a metallic or semiconductor electrode, sometimes by chemical reaction with surface groups and sometimes by adsorption. The attached substrate may be able to ligate, or it may be able to accept by exchange some electroactive species. Possibly some poetic licence will be allowed in defining such species since many interesting data have been obtained with ferrocene derivatives thus these organometallic compounds will be considered coordination compounds for the purpose of this chapter. 

Electrocatalytic Activity of Semiconductor Electrodes Modified by Surface-Deposited Metal Nanophase... 

Evidently, the adequate description of electrode impedance in the case of the actual contact between the surface-modified semiconductor and electrolyte represents a very complicated problem owing to the appearance of some hardly measured parameters... 

The Mott-Schottky plot obtained experimentally for the Ag-modified Ti02 electrode, which satisfy the above requirements, differs from that for the initial electrode by the slope value, with an insignificant shift of the point obtained after extrapolating the plot to the electrode potential axis (Fig. 6.15). Since for the realization of such electrode system we have used a semiconductor characterized by the high concentration of ionized donors, under consideration of Mott-Schottky dependence it is worthwhile to take account of the Helmholtz layer capacity (CH) placed in series with the space charge capacity [100] ... 

For the more accurate description of the Mott-Schottky dependences of semiconductor electrodes modified with small metal particles, it is reasonable to take into account the contribution of the capacity of electronic surface states (C ) induced by the... 

Ion-sensitive field effect transistor (ISFET) — In a semiconductor device based on the principle of the field effect transistor (FET) the current between two - semiconductor electrodes (designated source and drain) is controlled by a third electrode, the gate. In an ISFET this gate is modified on its surface in a way which makes the surface ion-responsive (-selective and -sensitive). Changes in the concentration of the species in the solution in contact with the gate surface thus control the current between source and drain. In order to establish proper working conditions a reference electrode (e.g., a -+ REFET) is needed. See also - CHEM-FET. 

Photoelectrochemical (PEC) reduction of CO2 with a p-type semiconductor electrode can be regarded as one of the solar energy conversion technologies and is important from a view-point of the global environmental problems. The reaction proceeds by essentially the same mechanism as photosynthesis and is of much interest as an artificial model for it. A number of studies have been made [1], but the photovoltage or the solar-to-chemical energy conversion efficiency still remains relatively low. We reported [2-4] that a p-Si electrode modified with small metal (Cu, Au and Ag) particles worked as an ideal-type electrode for the PEC reduction of CO2 in aqueous solutions. In the present paper we will report that the electrode of this type is also effective for the PEC reduction of CO2 in non-aqueous solutions which have high CO2 solubility. 

Semiconductor electrodes modified with reagents I-III exhibit properties that are fairly well predicted from the properties associated with the naked semiconductors in contact with ferrocene or Mv2+. Strongly interacting modifiers may alter the interface energetics and surface state distribution in useful ways.(11-14) A classic example of altering surface state distribution comes from electronic devices based on Si.(48) The semiconducting Si has a large density of surface states situated between the valence band and the conduction band. Oxidation of... 

We are investigating the effects of binding non-electroactive molecules to electrode surfaces. The attached layer will be sufficiently thin (ca. 1 monolayer) that electron transfer across the electrode/electrolyte interface will not be inhibited. However, other surface properties may be advantageously modified. For semiconductor electrodes, desirable changes include suppression of the photo-activated surface corrosion and shifts in the flatband potential. We are seeking to improve the performance of semiconductor liquid-junction solar cells by these means. 

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,... 

Figure 4. Various semiconductor electrodes, modified with monolayers (covalently attached) of phthalocyanine tethered to the electrode surface (a) or multilayers (adsorbed or sublimed) which aggregate to leave a semiporous surface layer (b) and a uniform phthalocyanine film leading to a p-type semiconductor layer adjacent to the n-type semiconductor substrate (c).

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