Electron Injection, Charge Recombination, and Energy Migration in Surface-Modified TiO₂ Nanocrystallite Layers. A Laser Photolysis Study

Abstract

Ru(bpy)3 2+ (bpy ≡ bipyridine) and Ru(o-phen)3 2+ (o-phen ≡ o-phenanthroline) have been adsorbed to a membrane layer made of TiO2 nanocrystallites from aqueous solutions at pH 2.5 by means of pretreatment of the surface with Nafion, sodium dodecyl sulfate (SDS) or sodium dodecyl benzyl sulfate (SDBS). Pulsed laser-induced emission and absorbance changes have been studied. The time profiles provide information concerning environmental effects (charge and hydrophobic interactions) on the rates and yields of electron injection from the excited dyes to the TiO2 membrane and subsequent electron recapture by Ru(III). The differences between the rates of electron injection and recapture, the multiexponential nature of these reactions, and the differences between specific photosensitizers and binders are discussed in light of the semiconducting properties of the TiO2 nanocrystallites and the hydrophobic and ionic interactions between the photosensitizers and the binders. Oxidation of iodide ions by Ru(III) was also studied. Iodide ions react efficiently with Ru(III) despite the negative charge of the binders, indicating that most of the charge is neutralized by the surface charge of TiO2 and by the Ru(III) ions. Quantum yields for net electron injection were determined from the initial (extrapolated) bleaching. In most systems observed bleaching corresponds to 30−80% of the absorbed photons. At relatively high laser pulse intensities, emission measurements show that bimolecular (and apparent higher order) processes take place, involving fast triplet annihilation. A detailed mechanism provides quantitative kinetic treatment of the data. Comparison of results in dry and wet layers indicates that energy migration is responsible for the enhanced triplet annihilation.

A surface science perspective on TiO2 photocatalysis

“…While much discussion in the literature has focused on optimizing the properties of the dye and the redox couples, attention here will be on the properties of TiO 2 as an electron acceptor. Coupling between the excited dye's electronic structure and the TiO 2 CB states is so strong that injection yields approaching 100% have been measured for many different dyes [610,612,622,623,631,633,636,638,640,652]. Another reflection of the strong coupling is that electron injection typically occurs on the sub-picosecond timescale [390,494,601,613,624,628,629,637,638,640,641,643,647,667,669], and in some cases faster than 100 fs [163,198,391,532,536,606,607,609,617,618,625,632,[649][650][651]655,656,662,666].…”

Relative Energetics at the Semiconductor/Sensitizing Dye/Electrolyte Interface

“…The dye-sensitization process is based on charge separation at the semiconductor/dye/electrolyte interface . Fast electron injection from the dye to the semiconductor occurs upon illumination followed by hole transfer to the electrolyte solution (regeneration of the dye). − Many factors, such as the distance between the dye and the semiconductor, the type of bond that connects them , and the properties of the redox couple in the electrolyte solution, influence the efficiency of the process.…”

Electron-Transfer Dynamics in DTDCI/MoS2 and DTDCI/WS2 Nanoclusters

“…Electron transfer from excited-state dyes into nanocrystalline semiconductors (specifically TiO 2 ) and semiconductors particles has recently received a great deal of attention. 1,2,[4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Most studies have found that this process occurs very quicklys typically in less than a picosecondsalthough in some cases, slower rates are observed. 18 Rapid ET is explained in terms of the large density of acceptor states in the semiconductor conduction band.…”

“…Electron transfer from excited-state dyes into nanocrystalline semiconductors (specifically TiO 2 ) and semiconductors particles has recently received a great deal of attention. ,,− Most studies have found that this process occurs very quicklytypically in less than a picosecondalthough in some cases, slower rates are observed . Rapid ET is explained in terms of the large density of acceptor states in the semiconductor conduction band. , Although of equal importance in determining the efficiency of charge separation, far less attention has been given to the dynamics of reverse ET.…”