Silicon redox reactions

To prepare pure silicon, silica is first converted to SiClq. A redox reaction between coke and chlorine gas is coupled with a metathesis reaction to give SiClq, which liquefies on cooling ... 

The redox reaction above can be used to determine the change in oxidation numbers for chlorine and silicon. According to this equation, what changes occurred ... 

Although halosilanes undergo smooth nucleophilic substitution at the silicon, they are rather inert to redox reactions. Therefore, chlorosilanes are usually used as trapping agents of anionic intermediates generated by electroreduction of organic compounds. However, in the absence of other reactive substrates halosilanes are reduced electrochemically to form Si-Si bonds. Indeed, there are a number of reports in the literature of the cathodic reduction of chlorosilanes (Sect. 4.2). 

Unstabilized chrome yellow pigments have poor lightfastness, and darken due to redox reactions. Recent developments have led to improvements in the fastness properties of chrome yellow pigments, especially toward sulfur dioxide and temperature. This has been achieved by coating the pigment particles with compounds of titanium, cerium, aluminum, antimony, and silicon [3.134] — [3.142]. 

While silicon tetrahalides also form adducts with amine N-oxides (Me3NO, pyO or bipyO)134, the reaction of Si2Clg with bipyO results in a redox reaction under cleavage of the Si—Si bond135 (equation 73). 

Participation of surface states may be important in the reactions of redox couples. It can result in the redox reaction to show an i-V curve with two one-electron waves. Such i-V curves are characteristic of metallic behavior rather than one two-electron wave as is expected at a semiconductor surface free of surface states. Metalliclike behavior may also be due to the interaction of redox species with silicon electrode, which may result in changes in the barrier height or cause chemical changes to form an ohmic-like interface. 

Deposition of a small amount of noble metals such as Cu, Pt, and Au increases the kinetics of redox reactions on silicon electrodes as shown in Fig. 6.3. Deposition of equivalent of 1 to 10 monolayers of Pt on silicon surface results in a shift of about 0.2V of the onset potential for hydrogen evolution to the positive direction. Because the flatband potential does not change with the Pt deposition, the enhanced hydrogen reaction kinetics is due to the catalytic effect of the deposited metal. The energy levels of the deposited metal grains are considered to lie in the middle of the band gap and communicate favorably to the surface states both energetically and spatially. The photovoltage of n-Si coated with sparsely scattered Pt islands has been found to increase proportionally to the inaease in the potential of the redox couple. Noble metal islands effectively collect photoelectrons and thus prevent the oxidation of the silicon surface by the photoelectrons. 

The surface condition—whether a native oxide is present or not— has a significant effect on bonding of iron and copper deposited on the surface and thus the cleaning efficiency. Each metal has its own characteristic chemical state. Metal impurity such as Fe tends to be present on the silicon surface oxide layer in the form of oxidized Fe(III), whereas Cu tends to bond directly to Si atoms in the elemental state. The former can be cleaned by etching away the oxide whereas the latter can only be removed by a redox reaction. 

The template removal step, needed to achieve porous materials, is one of the most critical points. In contrast to silica, other compositions are usually more sensitive to thermal treatments and calcination can result in breakdown of the mesostructures. Hydrolysis, redox reactions, or phase transfonnarions to the thermodynamically preferred denser crystalline phases account for this lower thermal stability. Many of the transition metal-based mesostruetured materials synthesized in the presence of cationic surfactants collapse during thermal treatments. The poor thermal stability observed could be due to the different 0x0 chemistry of the metals compared to silicon. Several oxidation states of the metal centers may be responsible for oxidation and/or reduction during calcination. In addition, incomplete condensation of the framewoik is possible. 

Ellingham diagrams enable us to discover, at a given temperature, the direction of the reaction between a metal and an oxide of a different metal. By way of example, let us consider the redox reaction between silicon and chrome oxide, which would be written as ... 

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