The role of electron transfer in photocatalysis: Fact and fictions
Graphical abstract
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Highlights
► Electron transfer plays a key role in photocatalysis. ► Electron transfer dynamics measured by flash photolysis, pulse radiolysis or stopped flow. ► Electron transfer efficiency depends on photocatalyst size, surface composition and morphology. ► The electron transfer rate was found to increase with increasing the driving force. ► New findings on the electron transfer reactions at TiO2 nanoparticles are reviewed.
Introduction
Photocatalysis using semiconductor nanoparticles have been the subject of research for many decades [1], [2], [3], [4], [5], [6], [7]. Recently, nanomaterials have attracted considerable renewed attention because of their promise in many current and potential applications such as electronic devises, sensors and catalysts [8], [9], [10], [11], [12], [13]. Depending on where the initial photo-excitation occurs, photocatalysis can be generally divided into two classes of processes. When the initial photo-excitation takes place on the catalyst and the photoexcited catalyst then transfers an electron or energy into a ground state molecule, the process is referred to as a catalyzed photoreaction (Fig. 1(a)). When the initial photoexcitation occurs in an adsorbate molecule (e.g., dye molecules) which then interacts with the ground state catalyst, the process is referred to as a sensitized photoreaction (Fig. 1(b)). The initial excitation of the system is followed by subsequent electron transfer and/or energy transfer.
The principle of the catalyzed photoreactions on semiconductors nanoparticles has been investigated over the past 20 years [1], [2], [3], [4], [5], [6], [7], [14]. Once electron/hole pairs are generated by light excitation, most of them recombine generating heat. Only a small fraction can successfully transfers to the interface to initiate redox reactions. The time scale of the interfacial electron transfer and the life time of photogenerated charge carrier determine the efficiency of photocatalytic processes. Electron photogeneration and recombination and electron transport are the elementary processes accounting for the conversion of light energy into useful charge carriers. Hence, increasing the efficiency of charge separation/transport in semiconductor nanoparticles is one of the major problems in photocatalysis to be addressed.
Charge carrier transfer has been widely acclaimed to be very important as it plays a pivotal role in photocatalytic processes. The fundamental study of the dynamics of the charge transfer processes is important in the design of new photocatalysts as well as for the industrialization of the photocatalytic processes. Electron transfer process is of great importance for the performance of electronic devices such as solar cells and sensors as well as in the photocatalytic processes such as photosynthesis and water remediation of organic (e.g., dye compounds, phenolic compounds) and inorganic contaminants (e.g., toxic metal ions).
A use of time resolved spectroscopy covering the time scale from picoseconds to millisecond and spectral range in the UV and visible light is a powerful tool to study the light induced electron transfer processes and provide mechanistic information voluble for design of novel and optimized materials. During the past decades, extensive investigations have been performed concerning the study of the kinetics and of the mechanistic details of the interfacial electron transfer processes at the semiconductor/water interface using the laser photolysis [14], [15], [16] and the pulse radiolysis technique [17], [18], [19], [20]. Recently, stopped flow technique has been employed to study the dynamics of one and multi-electron transfer reactions at TiO2 nanoparticles [21], [22], [23], [24].
In this article an overview on the charge carrier dynamics at semiconductor nanoparticles will be provided. The experimental techniques for nanoparticles synthesis, characterization as well as the development in the measurements of interfacial electron transfer dynamics properties will be reviewed. The factors affecting the efficiency of the electron transfer property including the nanoparticle surface, size and morphology will be discussed in detail. Of particular interest will be the new findings in dynamic study of some important electron transfer reactions at TiO2 nanoparticles. Comparison and contrast will be made between different employed methods. A future direction and a summary will be given at the end of the article.
Section snippets
Dynamics of charge carrier trapping and recombination
Exciting a semiconductor with any light source causes interband transition, excitonic transition or below-band gap transitions. If the photon energy is larger than the band gap, interband transitions dominate, exciting electrons from the valence band into the conduction band in the fs timescale (Eq. (1)). Tamaki et al. [25] have proposed that the electrons and holes walk randomly to the surface of the photocatalyst and are trapped there in the sub-ps timescale (Eq. (2a)). Electron and hole can
Synthesis and characterization of semiconductor nanoparticles
There are several methods to prepare semiconductor particles ranging from simple chemical method such as sol–gel method to mechanical and vacuum method. The experimental details of the preparation methods are not of our interest in this article. However, it is important to specify some limitations of using semiconductor nanoparticles in aqueous solution for the performing the spectroscopic dynamic studies. For absorption measurements it is important to use transparent colloidal suspension, so
Factors affecting electron transfer efficiency
The key to improve photocatalytic activity depends on the photocatalyst size [75], surface composition [76], and morphology [77]. There have not been yet systematic studies on the effects of the particle size and morphology on the electron transfer efficiency. This is largely because of the difficulty of preparing single-size, single-shape particles with uniform and well defined surface properties. However, the results reported to date suggest that the efficiency of electron transfer reactions
Reduction of viologen compounds
Viologen compounds were chosen as model electron acceptors to study the interfacial electron transfer at semiconductor nanoparticles [40], [16], [121], [122]. Viologen compounds (N,N′-dialkyl-4,4′-dipyridinium dichloride) are well known to be subjected to one-electron reduction which produces the blue radical cation (V+) from the colorless dication (V2+) (Eq. (6)). Viologen radical cations have attracted much attention because of their strong redox ability. They are capable of reducing protons
Relation between interfacial electron transfer rate constants and driving force
One of the most important parameters that affects the efficiency of the electron transfer reactions is the standard redox potential of the involved electron acceptor related to the standard redox potential of the conduction band electron because only those species with reduction potentials much more positive than the conduction band edge can be photoreduced.
The difference between the standard redox potential of the electron acceptor and the standard redox potential of the
Future challenges and prospects
The main critical limitation to be achieved in photocatalysis is to increase of the efficiency of charge separation/transport in semiconductor nanoparticles. Although, several attempts have been made to improve the charge transfer efficiency including metal ions doping, surface modification and semiconductor coupling, the strategy to solve this problem need to be change. Firstly, the factors determining the electron transfer activity should be identified and a subsequent verification of how
Summary
Electron transfer process plays a significant role in photocatalysis. The fundamental study of the dynamics of the charge transfer processes is important in the design of new photocatalysts as well as in the industrialization of the photocatalytic processes. This review highlights some significant insights obtained from the dynamics of interfacial electron transfer in photocatalysis. A focus was on the measurement of the interfacial electron transfer dynamics comprising different techniques
Acknowledgements
Prof. Dr. Jean-Marie Hermann is gratefully acknowledged for his numerous contributions in the field of photocatalysis making this field attractive to many young scientists. Financial Support from Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged (Grant no. BA 1137/8-1).
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