Efficient, Rapid Photooxidation of Chemisorbed Polyhydroxyl Alcohols and Carbohydrates by TiO₂Nanoparticles in an Aqueous Solution

Abstract

Time-resolved transient absorption spectroscopy has been used to study nanosecond and sub-microsecond electron dynamics in aqueous anatase nanoparticles (pH=3-4, 4.6 nm diameter) in the presence of hole scavengers: chemisorbed polyols and carbohydrates. These polyhydroxylated compounds are rapidly oxidized by the holes; 50-60% of these holes are scavenged within the duration of 3.3 ns FWHM, 355 nm excitation laser pulse. The scavenging efficiency rapidly increases with the number of anchoring hydroxyl groups and varies considerably as a function of the carbohydrate structure. A specific binding site for the polyols and carbohydrates is suggested that involves an octahedral Ti atom chelated by the -CH 2 (OH)-CH 2 (OH)-ligand. This mode of binding accounts for the depletion of undercoordinated Ti atoms observed in the XANES spectra of polyol coated nanoparticles. We suggest that these binding sites trap a substantial fraction of holes before the latter descend to surface traps and/or recombine with free electrons. The resulting oxygen hole center rapidly loses a CH proton to the environment, yielding a metastable C-centered radical.

Hydrogen Production by Photoreforming of Renewable Substrates

“…They also serve as active sites for the adsorption of reactants and intermediates [201]. It was concluded by time-resolved transient absorption spectroscopy [202,203] that C2-C6 polyols and carbohydrates could scavenge 50-60% of the photogenerated holes of TiO 2 in the time lapse of 6 ns. The scavenging efficiency was higher for increasing the number of OH groups.…”

Photocatalytic Redox Reactions for In‐Source Peptide Fragmentation

“…Indeed, alcohol, glucose, and other sugars are commonly used to supply electrons or capture holes on TiO 2 in photoelectrochemistry. [16] Under the UV radiation, the glucose can inject electrons to the valence band of the TiO 2 and many oxidative radicals, for example, [RCH 2 OC], [RCOOC] and [RCOC] can be generated on the surface of the mesoporous structure, [27] which can either undergo further in-source reactions with each other to form complexes or work as initial reactive species to oxidize other samples, for example peptides in this work. an ion extraction delay (600 ns).…”

“…It is supposed to be a consequence of the different electron-donating abilities of these reagents (see Supporting Information for further details). [16] In-source photooxidation-induced dissociation: To reach a stable state, the oxidative glucose radicals, [RCH 2 OC], [RCOOC] and [RCOC], generated on the TiO 2 mesoporous structure would either capture a hydrogen radical or an electron from the peptide, which would therefore be oxidized. As the fragmentation happens randomly between every two amino acids, the oxidation reaction probably takes place at the backbone of the peptides rather than at special side chains.…”

“…By depositing peptides and glucose on the mesoporous TiO 2 -modified photosensitive target plate, fragmentation was easily observed during laser desorption ionization (LDI) without the assistance of any conventional organic matrix. Here, glucose can be considered as a very efficient electron donor, [16] working as a hole scavenger or conductor to help either reduction on the surface of the TiO 2 layer or long-distance oxidation in the plume. After scavenging holes, the electron quenching from on-surface or in-volume recombination can be greatly prevented, [13] therefore releasing more free electrons for in-source reduction reactions, while the generated oxidized glucose can further oxidize other present species, thus realizing long-distance inplume oxidation reactions.…”

Analysis of Photocatalytic Reactions Using a TiO2 Immobilized Microreactor

“…Several types of radical species are known to be involved in the reaction, and their lifetime depends on conditions such as solution pH and temperature. [5][6][7][8] Photocatalytic processes are classified into direct [9][10][11][12] and secondary [13][14][15] reactions. Direct reactions occur between adsorbed species and photoexcited electrons or holes, while secondary reactions are induced by active oxygen species generated in the solvent after photoexcited electrons and holes react with solvent molecules.…”