Particulate semiconductors

Although the De Broglie wavelength of free electrons is 0.1 nm, the value of an electron in a small crystallite can be much larger because the effective mass of electrons in a small particle is considerably smaller. Energy levels evolve from HOMO and LUMO to those of clusters, Q-sized particles, and finally bulk semiconductor. Figure 7.7 shows the energy levels in bulk- and Q-sized particulate semiconductors. 

Energy levels in (a) bulk semiconductors, (b) Q-sized particulate semiconductors, and (c) semiconductor particle with metal islands. 

Lee JS (2006) Photocatalytic water splitting under visible light with particulate semiconductor catalysts. Catal Surv Asia 9 217-227... 

CdS, ZnS, PbS, CuS, and In2S3 System A = single composition of particulate semiconductor on one side of the BLM System B = two different compositions of particulate films on the same side of the BLM System C = two different compositions of particulate films on opposite sides of the BLM... 

Deposition of a particulate semiconductor on the cis side of the BLM (System A) alters the equivalent circuit to that shown in Fig. 108b, where Rf and... 

Fig. 108a-c. Proposed equivalent circuits for. a an empty and b a semiconductor-particle-coated BLM. Porous structure of the semiconductor particles allowed c the simplification of the equivalent circuit. Rm, RH, and Rsol are resistances due to the membrane, to the Helmholtz electrical double layer, and to the electrolyte solutions, while C and CH are the corresponding capacitances Rf and Cf are the resistance and capacitance due to the particulate semiconductor film R m and Cm are the resistance and capacitance of the parts of the BLM which remained unaltered by the incorporation of the semiconductor particles R and Csc are the space charge resistance and capacitance at the semiconductor particle-BLM interface and Rss and C are the resistance and capacitance due to surface-state on the semiconductor particles in the BLM [652]... 

Metals, such as platinum, are usually introduced to improve the electron-hole separation efficiency. In order to analyze the energy structure of the metal-loaded particulate semiconductor, we solved the two-dimensional Poisson-Boltzmann equation.3) When the metal is deposited to the semiconductor by, for example, evaporation, a Schottky barrier is usually formed.45 For the Schottky type contact, the barrier height increases with an increase of the work function of the metal,4 which should decrease the photocatalytic activity. However, higher activity was actually observed for the metal with a higher work function.55 This results from the fact that ohmic contact with deposited metal particles is established in photocatalysts when the deposited semiconductor is treated by heat65 or metal is deposited by the photocatalytic reaction.75 Therefore, in the numerical computation we assumed ohmic contact at the energy level junction of the metal and semiconductor. 

This has led to an increased interest in the elucidation of the mechanistic details of such reactions and the photocatalytic properties of particulate semiconductors for the purposes of either trying to improve process efficiency prior to engineering scale-up or, in the case of acid rain production, deliberate inhibition. Indeed, as has been noted by Ollis and Al-Ekabi [44] and more recently reiterated by Hoffmann et al. [45], the average publication rate over the last 10 years in the areas of water, air and wastewater treatment alone exceeds 200 papers per year. The publication of authoritative review articles on colloidal semiconductors over the last decade has been correspondingly prolific [44-68] and the interested reader is referred to them for information beyond the scope of this review. 

Thin-lilm photoelectrodes are needed in photoelectrocatalytic systems to apply a bias potential, either for the photoelectrode characterization or to facilitate the photocatalytic reactions. However, to be able to present a more comprehensive view on the performance of different materials, our subsequent discussions will focus on particulate semiconductor photocatalysts since the latter have been much more extensively investigated. Their electronic band structure (i.e., both the bandgap energy and the positions of CB and VB) is the key factor to determine whether or not a semiconductor material is suitable for a specific photocatalytic reaction, as will be demonstrated by reviewing a number of selected metal oxides and cou-pled/composite materials based on various semiconductors. 

In choosing suitable particulate semiconductor photo catalysts for use in nuclear fuel processing scenarios, it is necessary that certain performance criteria should be fulfilled. Suitable materials should have the following properties ... 

Electrons and holes that are generated in particulate semiconductors are localized at different defect sites on the surface and in the lattice of the particles. Electron paramagnetic resonance (EPR) results have shown that electrons are trapped as two reduced metal centers—Ti(III) sites—eoordinated either [38, 39] 1) with anatase lattice oxygen atoms only, or 2) with OH or H2O the holes are trapped as oxygen-centered radicals covalently linked to surface titanium atoms [40] (Figure 7). This is summarized by Eqs. (7)-(9). 

Nanostructured particulate semiconductor electrodes are a topic of current interest since they offer the possibility of cheap and efficient dye sensitised solar cells [20-26]. These cells rely on efficient light harvesting by a dye adsorbed on the high internal surface area of the electrode. Rapid electron injection from the excited dye into a Ti02 particle and subsequent regeneration of the dye from its oxidised state by... 

Especially in the case of the particulate semiconductors, there is now an extensive laboratory chemistry of the catalytic reactions which may be accomplished. However, the situation in a natural setting is much more complex than is characteristic of model studies. Before we begin a review of important reactions now known, it is useful to set out some comments on the factors, which differentiate the laboratory study from the natural system. 

Some particulate semiconductors catalyse photo-induced oxidative polymerization of pyrrole in the presence of an appropriate electron acceptor under their band-gap excitations. As a result, the surfaces of these semiconductors are covered with PPy [13], and the adhering PPy is effective as a hole carrier from the valence band in the semiconductor to any electrolytes [14]. Such phenomena have also been observed on ITO glass covered with polyaniline. 

Historically, nanoparticles were the first nanostructured photoelectrochemical motif to be explored and studied. A number of studies have been performed to assess the photocatalytic properties of many types of semiconductor (particularly metal oxide) nanoparticles as catalysts for oxidation reactions and purification/cleaning applications. Separately, the photoelectrode behaviors of compact, particulate semiconductor films have been analyzed. Such films serve as the backbone of the dye-sensitized photoelectrodes first popularized by O Regan and Gratzel. " Separately, semiconductor nanoparticles small enough to exhibit quantum-confinement effects have been used to prepare Schottky-type heterojunctions with the capacity for attaining values... 

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