Nanoparticle-based Photocatalytic Water Splitting

The following reactions are involved for the production of hydrogen and oxygen from water (pH = 7)  

When a semiconductor is irradiated by photons of energy equal to or greater than that of its bandgap, which it absorbs, excitation occurs and an electron moves to the conduction band leaving a hole behind in the valance band. For Ti02 this process is expressed as  

The photogenerated electrons and holes can recombine in bulk or on the semiconductor surface releasing energy in the form of heat or a photon. The electrons and holes that migrate to the semiconductor surface without recombination can, respectively, reduce and oxidize water (or the reactant) and are the basic mechanism of photocatalytic hydrogen production, see Fig. 6.3. 

There are various methods available for the synthesis of 1-100 nm diameter nanoparticles. Whatever the synthesis method, it is important to consider their stability in terms of composition and size. 

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The recent worldwide emphasis on the topics of energy conservation and energy production has directed increased attention on molecular based catalysts, in particular for photocatalytic water splitting. An example of the contributions that resonance Raman and SERS can potentially make to this broad area is found in the field of photocatalytic production of dihydrogen. These catalysts are typically based on a light harvesting moiety, for example a ruthenium complex and a catalytic centre, typically palladium-or platinum-based. A recurrent question in regard to the use of these catalysts is that of the decomposition of complexes to form potentially active nanoparticles. 

The following sections deals with the reactions studied within this work. For UHV and ambient experiments CO oxidation, for UHV ethene hydrogenation and for ambient conditions photocatalytic water splitting is introduced. As these model reactions are extensively studied in surface science only a brief overview is given. Further, the survey is limited to findings on Pt surfaces and Pt nanoparticles, in the case of photocatalysis, to semiconductor based systems (i.e. CdS). 

Photocatalytic systems based on the plasmon-induced charge separation can be used for oxidation of alcohols, aldehydes, and phenol [8, 13] mineralization of carboxylic acids [14] oxidation of benzene to phenol [15] release of hydrogen from alcohols and ammonia [16] and oxidation and reduction of water (but not water splitting) [17]. The photocatalytic system can also be applied to hydrophilic/hydrophobic patterning based on photocatalytic removal of a hydrophobic thiol adsorbed on metal nanoparticles [18]. 

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