ABSTRACT
Nanoparticles in Microemulsion ............................................... 487 18.4 Electron Transfer Dynamics in Catechol-Sensitized
TiO2 Nanoparticles ................................................................................. 489 18.4.1 Preparation of Semiconductor Colloids in Reverse Micelles .... 489 18.4.2 Dye Adsorption on TiO2 Colloids in Microemulsion
and Water ................................................................................... 490 18.4.3 Optical Absorption Spectroscopy of Nanoparticles
and Interaction with Dye Molecules ......................................... 490 18.4.4 Transient Absorption Measurements in Microemulsion ........... 492
18.4.5 Effect of Microenvironment in Interfacial Electron Transfer Dynamics ..................................................... 495
18.5 Conclusion .............................................................................................. 496 Acknowledgments ............................................................................................. 497 Symbols and Terminologies .............................................................................. 497 References ......................................................................................................... 497
Nanocrystalline semiconductor materials [1] exhibit a wide range of novel chemical and physical properties that are fi nding applications in devices such as solar cells [2], waste water treatment [3], and nano-electronic devices [4]. Dye sensitization of wide band-gap semiconductor electrodes has gained suffi cient attention in recent years, largely owing to the demonstration of dye-sensitized solar cells with a conversion effi ciency as high as 10% [2]. In nanocrystalline materials, a signifi cant fraction of the atoms reside on the nanocluster surface. These surface atoms having “dangling bonds” that may act as electron and hole traps that can dominate electron/hole recombination and other processes. It is often possible (and desirable) to passivate the surface traps. Several studies have shown that passivation of surface traps has large effects on the nanocluster photophysics [5-7]. These surface states also can take part in the interfacial electron transfer (IET) reaction. We have reported earlier that surface states of wide band-gap materials like ZrO2 can take part in the IET processes [8-10]. Presence of surface states in the nanostructured materials actually brings down the effi ciency of the devices. To gain higher efficiency in the devices it is very important to pacify these surface states. Modifi cation of these states is possible using suitable modifi er molecules. By this process it is possible to remove most of the lower lying surface states. Surface modifi cation of semiconductor NPs changes their optical, chemical, and photocatalytic properties signifi cantly [11]. It can lead to the following effects: (1) it may enhance their excitonic and defect emission by blocking nonradiative electron/ hole (e−/h+) recombination at the defect sites (traps) on the surface of the semiconductor NPs [5]; (2) it may enhance the photostability of semiconductor NPs [5]; (3) it may create new traps on the surface of the NPs leading to the appearance of new emission bands [12]; and (4) it may enhance the selectivity and effi ciency of light-induced reactions occurring on the surface of semiconductor NPs [11,13]. On surface modifi cation, density of surface states (lower lying states below the conduction band) can be changed drastically. We have observed that on surface modifi cation the optical and photochemical properties of NP changed [14]. Electron injection dynamics has been found unaffected by surface modifi cation; however, back electron transfer (BET) dynamics is found to be slow on modifi ed surface as compared to that on bare surface [15,16], which in turn can increase the effi ciency of solar cell.
