Polymer coated semiconductor

Since photoinduced charge separation occurs easily at the semiconductor-water interface, its stabilization with polymer coating is one of the most promising approaches to water photolysis by visible irradiation. 

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Fig. 19. Water photolysis by polymer coated semiconductor of narrow bandgap...

Many of the earliest studies focused on the use of polymer-coated semiconductor materials for the reduction of C02. An example was the study of Aurian-Blajeni et al., who electropolymerized polyaniline onto p-Si [116]. In an aqueous C02-saturated solution, a total faradaic efficiency for formic acid and formaldehyde of 28% was achieved, but at a potential of—1.9 V (versus SCE). Likewise, Cabrera and Abruna electropolymerized [Re(CO)3(v-bpy)Cl], where v-bpy is 4-vinyl-4 -methyl-2,2 -bipyridine [117]. For CO production, TONs of about 450 were observed, while the faradaic efficiencies approached 100%. Upon illumination in acetonitrile solution, the onset potential for reduction was -0.65 V (versus SCE). 

On a side note, Ouskova and co-workers also reported that the composite of magnetic /i-FejOs nanorods in 5CB showed lower threshold voltages than pure 5CB, and that the sensitivity of the nematic liquid crystal to external magnetic fields was increased in the presence of such magnetic nanorods [451]. Finally, several groups interested in the macroscopic organization and orientation of nanorods also reported on the formation of a lyotropic liquid crystal phase induced by the self-assembly of polymer-coated semiconductor nanorods [453—457], which might be used to improve the device performance, for example, of solar cells. 

Polyacrylamide gel films containing Fe(III) dipyridinium complex as an electrochromic component were usefiil as the substrate for the formation of photographic images when coated on a CdSe film [48]. Poly-Re (CO) 3 (vbpy)Cl-coa ted n-MoSc2 also allowed photoinduced electrochromic recordings in the coated polymer layer under anodic bias [49]. The images produced are easily erased by applying the reverse bias to the polymer-coated semiconductor electrodes. 

The above mechanistic aspect of electron transport in electroactive polymer films has been an active and chemically rich research topic (13-18) in polymer coated electrodes. We have called (19) the process "redox conduction", since it is a non-ohmic form of electrical conductivity that is intrinsically different from that in metals or semiconductors. Some of the special characteristics of redox conductivity are non-linear current-voltage relations and a narrow band of conductivity centered around electrode potentials that yield the necessary mixture of oxidized and reduced states of the redox sites in the polymer (mixed valent form). Electron hopping in redox conductivity is obviously also peculiar to polymers whose sites comprise spatially localized electronic states. 

Schwerzel, R.E. and Spahr, K.B., Hydrogen Production with Photoactive Semiconductor Catalysts Stabilized by Metallized Plasma Polymer Coatings, Final Report to U.S. Department of Energy, Brookhaven National Laboratory, Battelle, 1989. 

Quantum clusters are highly photostable when compared with organic fluorophores. A study was conducted to check the photostability of clusters in comparison to organic fluorophores and semiconductor quantum dots [12]. Photostability of a gold cluster capped with dihydrolipoic acid (AuNC DHLA) was compared with polymer coated CdSe/ZnS semiconductor quantum dots and two different organic fluorophores namely fluorescein and rhodamine 6G (Fig. 5). For the study, 20 pi of fluorescent AuNC DHLA was dissolved in sodium borate buffer of pH 9. The sample was loaded into a quartz cuvette and was exposed to blue-light (480 nm)... 

Fig. 5 Photostability of fluorescent Au nanoclusters (AuNC DHLA) compared with semiconductor quantum dots (polymer-coated QD 520 from Invitrogen) and organic fluorophores (fluorescein, rhodamine 6G) [12]...

If small bandgap semiconductors could be stabilized in water for much more prolonged time by polymer coating, it must lead to developments of efficient photochemical cell and water photolysis system. 

Not all these cited studies have focused on photodriven HER and OER applications for the coated semi-conductor/electrolyte interfaces. Ae.c.p.= electronically-conducting polymer, sc = semiconductor... 

Coating the surface of silicon electrodes with a polymer coating can also be an effective method of stabilizing the electrodes and improving the photovoltage and kinetics of electrode reactions " The polymer film effectively insulates the semiconductor from the superoxide ion and prevents chemical reaction and deterioration. At the same time, the polymer behaves like a surface-bond redox couple to mediate the charge transfer between the semiconductor and the redox species in the solution. Various types of polymers can be used to coat silicon electrodes as shown in Table 6.6. 

Chemical sensors for gas molecules may, in principle, monitor physisorp-tion, chemisorption, surface defects, grain boundaries or bulk defect reactions [40]. Several chemical sensors are available mass-sensitive sensors, conducting polymers and semiconductors. Mass-sensitive sensors include quartz resonators, piezoelectric sensors or surface acoustic wave sensors [41-43]. The basis is a quartz resonator coated with a sensing membrane which works as a chemical sensor. 

The photocorrosion of the semiconductor has been reduced by the polymer coating. The simultaneous generation of Hg is zm advantage of this process. This method gives a power conversion efficiency of 0.6% and a quantum efficiency of nearly 1%. 

A semlquantltatlve meaBurement technique based on the general principles of photographic sensltometry has been developed for relative evaluation of the photosensitivity of polymer coatings (58,59). Quantitative data were obtained with a technique developed on the basis of silicon semiconductor devices technology (60). 

PEC catalysis using polymeric assistance is considered to be a natural outgrowth of polymer assisted electrochemical and photochemical catalysis. Early experiments, mostly combining electroactive polymer films with small band gap semiconductors have demonstrated the feasibility of using such systems in the PEC decomposition of water. Both gaseous H2 and O2 have been separately generated from polymer coated photoelectrodes H2 from poly-Mv2+ films on p-Si and O2 from polypyrrole films on n-CdS. 

The major success of polymer coated photoanodes is to protect them against photooxidation, which rapidly degrades the performance of many small band gap n-type semiconductors. 

In this chapter, we will describe and discuss the fabrication and characterization of metal oxide thick- and thin-film sensors (which are based on the conductivity modulation principle) and SAW devices (which are based on frequency modulation). The sensing properties of thick or thin semiconductor films with various components will be discussed, focusing on sensitivity and stability. In addition, SAW-type CWA sensors will be introduced for different pairs of inteidigitated transducer (IDT) fingers, various wave lengths, and different polymer coatings on the input and output IDTs of the device. 

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