Interface support-semiconductor

A large variety of organic oxidations, reductions, and rearrangements show photocatalysis at interfaces, usually of a semiconductor. The subject has been reviewed [326,327] some specific examples are the photo-Kolbe reaction (decarboxylation of acetic acid) using Pt supported on anatase [328], the pho-... 

On the other hand, Switzer et al. proposed a different model for the oscillation. They attributed the oscillation to repetitive build-up and breakdown of a thin CU2O layer, which is a p-type semiconductor and acts as a thin rectifying (passivating) layer [24]. Disappearance of the oscillation under irradiated condition supports this model. Light will generate electron-hole pairs in the CU2O and lower the rectifying barrier at the semiconductor/solution interface. 

The authors propose that a major difficulty in interpreting kinetic current flow at the semiconductor-solution interface lies in the inability of experimentalists to prepare interfaces with ideal and measurable properties. In support of this hypothesis, the importance of ideal interfacial properties to metal electrode kinetic studies is briefly reviewed and a set of criteria for ideality of semiconductor-solution interfaces is developed. Finally, the use of semiconducting metal dichalcogenide electrodes as ideal interfaces for subsequent kinetic studies is explored. 

In many catalytic systems, nanoscopic metallic particles are dispersed on ceramic supports and exhibit different stmctures and properties from bulk due to size effect and metal support interaction etc. For very small metal particles, particle size may influence both geometric and electronic structures. For example, gold particles may undergo a metal-semiconductor transition at the size of about 3.5 nm and become active in CO oxidation [10]. Lattice contractions have been observed in metals such as Pt and Pd, when the particle size is smaller than 2-3 nm [11, 12]. Metal support interaction may have drastic effects on the chemisorptive properties of the metal phase [13-15]. Therefore the stmctural features such as particles size and shape, surface stmcture and configuration of metal-substrate interface are of great importance since these features influence the electronic stmctures and hence the catalytic activities. Particle shapes and size distributions of supported metal catalysts were extensively studied by TEM [16-19]. Surface stmctures such as facets and steps were observed by high-resolution surface profile imaging [20-23]. Metal support interaction and other behaviours under various environments were discussed at atomic scale based on the relevant stmctural information accessible by means of TEM [24-29]. 

System installation in a permanent location may require a sample conditioning system featuring some degree of automation, such as automatic cleaning (the system illustrated above features such a system) and outlier sample collection and the need to interface to an existing control system process computer. The latter may require that the system operates with a standardized communications protocol, such as Modbus, for the chemical industry. Certain specialized industries use different protocols, such as the semiconductor industry, which uses SECS and SEC-11 protocols. A standardized approach designated the Universal Fieldbus is another method/protocol for process analyzers which is being supported by certain hardware manufacturers. 

It is, of course, well known that metal-semiconductor interfaces frequently have rectifier characteristics. It is significant, however, that this characteristic has been confirmed specifically for systems that have been used as inverse supported catalysts, including the system NiO on Ag described above as catalyst for CO-oxidation. In the experimental approach taken, nickel was evaporated onto a silver electrode and then oxidized in oxygen. A space charge-free counter-electrode was then evaporated onto the nickel oxide layer, and the resulting sandwich structure was annealed. The electrical characteristic of this structure is represented in Fig. 8. The abscissa (U) is the applied potential the ordi-... 

For the high voltage regime it is reasonable to suppose that a semiconductor of thickness L is contacted with two electrodes which, by virtue of a low energy barrier at the interface, are able to support the transport of an infinite number of one type of mobile carrier. The current will then become limited by its own space charge, and this can in the extreme case reduce the electric field at the injecting contact to zero. This is realized when the number of carriers per unit area inside the sample approaches the capacitor charge of the diode, i.e., eeo/e. This number of carriers can be transported per unit transit time ttT = L/fj,. 

A similar treatment can be used to calculate the electric field and the electric potential in the metal. However, in a metal, both the electric field and the electric potential drop to zero at a very short distance from the semiconductor/metal interface. This occurs because metals do not support electric fields, and all of the excess charge density resides on the surface of the metallic phase. The surface dipole layer is therefore effectively screened from test charges at any finite distance into the metallic phase, and the width of the electric potential gradient is extremely small. Because charge carriers can pass freely through this extremely thin barrier, only the electric field in the semiconductor significantly affects the electrical properties of semiconductor/metal contacts. 

A similar smface tension treatment can be made for the interface between metal oxides and metals. Native oxides typically have lower surface free energies than the bare metal, in turn driving the surface oxidation of most metals. However it is not true that all oxides have lower surface tensions than all metals or semiconductors. For the case of growth of a metal oxide film on a dissimilar metal, or a metal on a supporting metal oxide substrate, the initial phases of growth are determined by the respective surface tensions. These are described by the Young-Dupre equation... 

Photonic band gap (PEG) materials represent a class of composites that are designed to monitor the properties of photons in much the same manner as semiconductors manipulate the electrons properties. These composites have been developed from ordered arrays of self-assembled nanometer-sized polyst3a ene spheres that have been repeatedly encapsulated with various polymeric systems. However, it has been recognized that PEG materials also support surface electromagnetic waves-optical modes that propagate at the interface of a PEG crystal [193]. 

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