Semiconductor electrode band structure

In addition to the stoichiometry of the anodic oxide the knowledge about electronic and band structure properties is of importance for the understanding of electrochemical reactions and in situ optical data. As has been described above, valence band spectroscopy, preferably performed using UPS, provides information about the distribution of the density of electronic states close to the Fermi level and about the position of the valence band with respect to the Fermi level in the case of semiconductors. The UPS data for an anodic oxide film on a gold electrode in Fig. 17 clearly proves the semiconducting properties of the oxide with a band gap of roughly 1.6 eV (assuming n-type behaviour). 

Semiconductor electrodes seem to be attractive and promising materials for carbon dioxide reduction to highly reduced products such as methanol and methane, in contrast to many metal electrodes at which formic acid or CO is the major reduction product. This potential utility of semiconductor materials is due to their band structure (especially the conduction band level, where multielectron transfer may be achieved)76 and chemical properties (e.g., C02 is well known to adsorb onto metal oxides and/ or noble metal-doped metal oxides to become more active states77-81). Recently, several reports dealing with C02 reduction at n-type semiconductors in the dark have appeared, as described below. 

The electronic structure of the semiconductor electrodes is usually described in terms of energy bands that can effectively be considered a continuum of energy levels due to the small difference in energy between adjacent molecular orbitals [66,67]. The highest energy band comprised of occupied molecular orbitals is called the... 

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. 

The main differences between the kinetics of electron transfer at a metal and that at a semiconductor electrode originate in the difference in the density of free carriers. In a typical metal, the electrochemical potential or Fermi level, is located in a band. The free-electron density is huge, lO -lO cm . We discussed the metal/ solution interface in Section 4.5 it has a rather complicated structure with a double... 

It had not been realized until recently that electrocatalysis in the water decomposition processes at photoactive semiconductor electrodes was as important as the band-structure properties of the semiconductor material itself. However, it is clear that the effective voltage, beyond the 1.23 V limit, or 1.23 F - hv, required to photoelectrolyze water at some net rate will also be determined, as with metals, by the electrocatalytic properties of the semiconductor surfaces. 

With respect to electron transfer processes, semiconductor electrodes are different from their metal counterparts in two ways. First, the band structure, characterized by a band gap separating the conduction and valence band, will express... 

In its classic form, ECL is regarded as a solution-phase process, on the basis of both direct evidence (Problem 18.4) and the expectation that metal electrodes quench excited states (18, 19). The band structure of semiconductor electrodes sometimes removes the latter difficulty (see Section 18.2), and emission from excited states produced directly in heterogeneous charge transfer at semiconductors can occur (20-22). More recently, even surface films, such as monolayer assemblies and polymer-modified electrodes (Chapter... 

It is important to determine the conductivity and flat-band potential ( ft) of a photoelectrode before carrying out any photoelectrochemical experiments. These properties help to elucidate the band structure of a semiconductor which ultimately determines its ability to drive efficient water splitting. Photoanodes (n-type conductivity) drive the oxygen evolution reaction (OER) at the electrode-electrolyte interface, while photocathodes (p-type conductivity) drive the hydrogen evolution reaction (HER). The conductivity type is determined from the direction of the shift in the open circuit potential upon illumination. Illuminating the electrode surface will shift the Fermi level of the bulk (measured potential) towards more anodic potentials for a p-type material and towards more cathodic potentials for a n-type material. The conductivity type is also used to determine the potential ranges for three-electrode j-V measurements (see section Three-Electrode J-V and Photocurrent Onset ) and type of suitable electrolyte solutions (see section Cell Setup and Connections for Three- and Two-Electrode Configurations ) used for the electrochemical analyses. 

The Efb is a property of the semiconductor interface that depends on the electrolyte in which the measurement is made. The onset of photocurrent does not necessarily define the potential because other interfacial effects may delay the onset to a point beyond the transition from accumulation to depletion. The error from such interfacial effects could be on the order of a few millivolts to over a volt. One such interfacial effect might be the kinetic overpotential required to drive the reaction. This overpotential shifts the photocurrent onset in the cathodic direction for p-type samples and in the anodic direction for n-type samples. Therefore, catalysts are often deposited onto the electrode surface to minimize the overpotentials (see section Catalyst Surface Treatments ). However, the modification of electrode surfaces with catalysts may influence the semiconductor/electrolyte junction and surface states and additionally shift the Efb in unexpected ways. Ideally, the catalyst treatment will improve the accuracy of the measured by this technique, although effects such as Fermi level pinning may introduce a change in the band structure at the surface which may negate the improvement from a reduced kinetic overpotential. 

Potential-modulated UV-vis reflectance spectroscopy, often referred to as electroreflectance (ER), was originally developed in solid-state physics to characterize surfaces and was applied to studies of the electronic band structure of semiconductors. The ER technique has also been used to characterize metal electrode surfaces in the absence and presence of adsorbates. The reflectivity of metal electrodes is a function of the surface charge density of the electrodes. ER technique has also been used to investigate electrode reactions of organic species adsorbed on the electrode surfaces. Several review articles on ER are available [21-24]. 

Specular reflection spectroscopy has been actively used in in situ studies of the formation and optical behaviour of monolayer films on surfaces, and for detecting intermediates and products of heterogeneous chemical and electrochemical reactions. The vibrational spectra of the adsorbed species at electrode surfaces are obtained by surface-enhanced Raman scattering and infrared reflectance spectroscopies. Since the mid-1960s, modulated reflection spectroscopy techniques have been employed in elucidating the optical properties and band structure of solids. In the semiconductor electroreflectance, the reflectance change at the semiconductor surface caused by the perturbation of the dielectric properties of... 

Electrochemistry of semiconductor electrode is quite different from that of metal electrode as their electronic structures are different, and it is important to understand band stmctures of metal, semiconductor, and insulator as described below [1]. 

After band structures of metal, insulator, and semiconductors are described and historical back-grotmd of semiconductor electrochemistry is presented, electronic structure of semiconductor/ electrolyte solution interface is discussed in relation to the unique electrochemical behavior of semiconductor electrode. Finally, effect of illumination as well as the surface modification on the electrochemical behavior of semiconductor electrode are described. Fundamental knowledge of semiconductor electrode presented here should be very important for the future development of photoelectrochemical and photocatalytic energy... 

The properties of semiconductor electrodes and their differences compared with metal electrodes can be understood by examining their electronic structure. Semiconductors are unique in their electronic properties due to their band structure. The origin of the energy bands is generally discussed using band or zone theory, where the motion of a single electron in the crystal lattice is considered. It is assumed that there is no interaction between individual electrons or between the electrons and lattice points (the potential energy is zero). Therefore, the model is sometimes referred to as the nearly free electron model. 

To obtain information about the energetics of semiconductor electrodes using CV, it is not necessary to have a series of redox couples whose are distributed in the entire band structure. In fact, one can pinpoint the flat-band position by recording CVs for just one redox couple having two-electron systems, AVA and A VA with respective redox potentials o(i) and f o(2) such that these potentials lie above and below Eq. The simation is sketched in Figure 9.22. 

Consequently, excited electrons are penetrated into the semiconductor conduction band. Generally, DSSC structures consist of a photoelectrode, photosensitizer dye, a redox electrol5 e, and a counter electrode. Photoelectrodes could be made of materials such as metal oxide semiconductors. 

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