Interface energetics

TABLE 18 Data on the Interface Energetics of Sodium Alkanesulfonates on Tristearin (tristearin/5 x 103 mol 1 alkanesulfonate in distilled water at 50°C)... 

Aruchamy A, Wrighton MS (1980) Comparison of the interface energetics for n-type cadmium sulfide/ and cadmium teUuride/nonaqueous electrolyte junctions. J Phys Chem 84 2848-2854... 

Schneemeyer LF, Wrighton MS (1979) Flat-band potential of n-type semiconducting molybdenum disulfide by cyclic voltammetry of two-electron reductants Interface energetics and the sustained photooxidation of chloride. J Am Chem Soc 101 6496-6500... 

Schemes II and III represent the equilibrium interface energetics for ideal n- and p-type semiconductors, respectively, for...

Scheme II. Representation of the interface energetics for an ideal n-type semiconductor in contact with electrolyte solutions of differing Eredox.

The WS2 electrodes represent n- and p-type semiconductors that behave relatively ideally (15,16) with respect to interface energetics in that the value of Ey does vary with Ere(jox according to equation (1) for Erenox within 0.8 V of Epg. 

Scheme IV. Representation of the Is /T mediated oxidation of S02 at illuminated metal dichalcogenide photoanodes (top) and interface energetics with and without If/I in 6 M H2SOh/l M SOt for MoS2 (bottom) (31). In the absence of the mediator system the photovoltage for the S02 oxidation is expected to be negligible. The adsorption of the Is /T is unaffected by the S02 so the negative shift of the fiat band potential can be exploited to give a photovoltage for the desired process with the h /T system simultaneously providing an acceleration of the S02 oxidation.

Scheme VI. Representation of the interface energetics for intrinsic a-Si H at short circuit, dark equilibrium with ferricenium/ferrocene in EtOH, electrolyte solution (left) and under illumination with 632.8-nm light with a load in series in the external circuit (right). The diagrams are adapted from data in Reference 65 for intrinsic a-Si H (1-4-fi thick) on stainless steel first coated with heavily n-doped a-Si H (200-A thick) to ensure an ohmic contact near the bottom of the conduction band. In typical experiments Eredox = 0.4 V vs. SCE.

An important hypothesis related to interface energetics is that of Nyilas, who said that the free energy of adsorption basically drives conformational change 132>. He probed this approach by measuring the enthalpy of adsorption using micro calorimetry and attempted to relate surfaces with low heats of protein adsorption with increased blood compatibility. 

Scheme I Interface energetics for an n-type Si photoanode at the flat-band condition showing the formal potential for a surface-confined ferricenium/ferrocene reagent relative to the position of the top of the valence band, E ,and the bottom of the conduction band,E , at the interface between the Si substrate and the redox/electrolyte system. Interface energetics apply to an EtOH/0.1 M [n-Bu N]C10 electrolyte system.

A silicon electrode with a small amount of deposited noble metals such as Pt does not behave as a buried junction and the photoelectrode properties are still determined by contact of the electrolyte, not the metal. The photovoltaic response of the Pt-deposited silicon electrode is due to the incomplete coverage of the surface by Pt so that the interface energetics remains dominated by the contact of the semiconductor surface with the electrolyte. The mechanism of the hydrogen reduction processes involves first photoexcitation of electron-hole pairs followed by charging of the small Pt islands with excited electrons. The charged Pt islands react with H2O at a high rate. 

Highlights of research results from the chemical derivatization of n-type semiconductors with (1,1 -ferrocenediyl)dimethylsilane, , and its dichloro analogue, II, and from the derivatization of p-type semiconductors with N,N -bis[3-trimethoxysilyl)-propyl]-4,4 -bipyridinium dibromide, III are presented. Research shows that molecular derivatization with II can be used to suppress photo-anodic corrosion of n-type Si derivatization of p-type Si with III can be used to improve photoreduction kinetics for horseheart ferricyto-chrome c derivatization of p-type Si with III followed by incorporation of Pt(0) improves photoelectrochemical H2 production efficiency. Strongly interacting reagents can alter semicon-ductor/electrolyte interface energetics and surface state distributions as illustrated by n-type WS2/I-interactions and by differing etch procedures for n-type CdTe. 

Scheme I. Interface energetics for an n-type semiconductor under illumination giving an uphill oxidation of A to A+ to the extent of of Ey. Generally, the desired oxidation is only competitive with the anodic decomposition of the semiconductor. In the diagram Ef represents the electrode potential Eqb the bottom of the conduction band and Eyu the top of the valence band. At open-circuit Ey Eg,...

Alteration of Interface Energetics and Surface States by Chemical Modification... 

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... 

Scheme VIII. Interface energetics for n-type WS2 In the absence (a) and presence of I" in H2SO4/SO2 solution. Data are from ref. 11.

Aruchamy A. and Wrighton M. S. (1980), A comparison of the interface energetics for p-type cadmium sulfide-nonaqueous and cadmium telluride-nonaqueous electrolyte junctions , 7. Phys. Chem. 84, 2848-2854. 

Lin M. S., Hung N. and Wrighton M. S. (1982), Interface energetics for n-type semiconducting strontium titanate and titanium dioxide contacting liquid electrolyte solutions and competitive photoanodic decomposition in non-aqueous solutions , J. Electroanal. Chem. 135, 121-143. 

K. Mullen, and J. P. Rabe, Influence of molecular conformation on organic/ metal interface energetics, Chem. 

D. Gal et al., Engineering the Interface Energetics of Solar Cells by Grafting Molecular Properties onto Semiconductors, Proc. Indian Acad. Sci. 1997, 109, 487-496. 

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