Nanocrystalline systems

One of the most attractive features of colloidal semiconductor systems is the ability to control the mean particle size and size distribution by judicious choice of experimental conditions (such as reactant concentration, mixing regimen, reaction temperature, type of stabilizer, solvent composition, pH) during particle synthesis. Over the last decade and a half, innovative chemical [69], colloid chemical [69-72] and electrochemical [73-75] methods have been developed for the preparation of relatively monodispersed ultrasmall semiconductor particles. Such particles (typically 10 nm across [50, 59, 60]) are found to exhibit quantum effects when the particle radius becomes smaller than the Bohr radius of the first exciton state. Under this condition, the wave functions associated with photogenerated charge carriers within the particle (vide infra) are subject to extreme 

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N. J. Shaw, Aspects of the photoelectrochemistry of nanocrystalline systems, Electrochim. Acta 45 (1999) 549-560. 

Hagfeldt, A. Gratzel, M., Light-induced redox reactions in nanocrystalline systems. Chem. Rev. 1995, 95, 49-68. 

Hagfeldt A, Gratzel M (1995) Light-Induced Redox Reactions in Nanocrystalline Systems Chem Rev 95 49-68... 

To explanain photocatalytic and photoelectrochemical behaviour of such nanocrystalline systems one should take into consideration existence of various types of metal-doped Ti02 and Zr02 nanoparticles with different structural forms and spatial distribution of metal centers on their surface. 

Fig. 10.28. Model of charge carrier separation and charge transport in a nanocrystalline film. The electrolyte has contact with the individual nanocrystallites. Illumination produces an electron-hole pair in one crystallite. The hole transfers to the electrolyte and the electron traverses several crystallites before reaching the substrate. Note that the photogenerated hole always has a short distance (about the radius of the particle) to pass before reaching the semiconductor/electrolyte interface wherever the electron-hole pair is created in the nanoporous film. The probability for the electron to recombine will, however, depend on the distance between the photoexcited particle and the tin-coated oxide back-contact. (Reprinted with permission from A. Hagfeldt and Michael Gratzel, Light-Induced Redox Reactions in Nanocrystalline Systems Chem. Rev. 95 49-68, copyright 1995, American Chemical Society.)...

Charge transport in nanocrystalline electrodes is clearly strongly influenced by the inter-penetration of the solid and liquid phases. If electron hole pairs are generated by band to band excitation, it is usually observed that one type of carrier is transferred to the solution, while the other is transported to the substrate contact. In the case of the dye sensitized nanocrystalline systems, an electron is injected into the conduction band from the photoexcited dye and is then transported to the substrate. The dye is regenerated by reaction of its oxidised state with a supersen-sitiser such as 1 as shown in Fig. 8.25. 

Nanocrystalline systems display a number of unusual features that are not fully understood at present. In particular, further work is needed to clarify the relationship between carrier transport, trapping, inter-particle tunnelling and electron-electrolyte interactions in three dimensional nan-oporous systems. The photocurrent response of nanocrystalline electrodes is nonlinear, and the measured properties such as electron lifetime and diffusion coefficient are intensity dependent quantities. Intensity dependent trap occupation may provide an explanation for this behaviour, and methods for distinguishing between trapped and mobile electrons, for example optically, are needed. Most models of electron transport make a priori assumptions that diffusion dominates because the internal electric fields are small. However, field assisted electron transport may also contribute to the measured photocurrent response, and this question needs to be addressed in future work. 

Hagfeldt A., Gratzel M. (1995) Light-Induced redox-Reactions in Nanocrystalline Systems, Chem. Rev. 95(1), 49-68. 

Photoemission experiments that probe the electronic structure of the nanocrystals are indispensable if one wishes to gain insight into the electronic structure-property relationship. Though very few studies have been carried out on semiconducting nanocrystalline systems to date, techniques such as photoemission and X-ray absorption spectroscopies are of immense value in probing the electronic structure and also in verifying various theories proposed for the nanocrystals. In Section 11.6 we discuss these spectroscopic studies. 

The second factor can lead to nanocrystals adopting different morphologies to bulk crystals, with different exposed lattice planes leading to an extraordinary surface chemistry and catalytic activity [14]. The importance of surfaces and boundaries in nanocrystalline systems is demonstrated in Figure 4.1, which shows the fraction of atoms in these regions as a function of grain size. 

The dynamics of charge transfer and back reaction in dye sensitised nanocrystalline systems such as Sn02 have also been studied extensively by transient optical and microwave absorption spectroscopies [193-199],... 

Another example of the influence of nanometric effects is on the Debye temperature, which decreases in nanocrystalline systems, and is lower when the same... 

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