Imogolite nanotubes are potentially promising co-photocatalysts because they are predicted to have curvature-induced, efficient electron-hole pair separation. This prediction has however not yet been experimentally proven. Here, we investigated the behavior upon irradiation of these inorganic nanotubes as a function of their water content to understand the fate of the generated electrons and holes. Two types of aluminosilicate nanotubes were studied: one was hydrophilic on its external and internal surfaces (IMO-OH) and the other had a hydrophobic internal cavity due to Si-CH3 bonds (IMO-CH3), with the external surface remaining hydrophilic. Picosecond pulse radiolysis experiments demonstrated that the electrons are efficiently driven outward. For imogolite samples with very few external water molecules (around 1% of the total mass), quasi-free electrons were formed. They were able to attach to a water molecule, generating a water radical anion, which ultimately led to dihydrogen. When more external water molecules were present, solvated electrons, precursors of dihydrogen, were formed. In contrast, holes moved towards the internal surface of the tubes. They mainly led to the formation of dihydrogen and of methane in irradiated IMO-CH3. The attachment of the quasi-free electron to water was a very efficient process and accounted for the high dihydrogen production at low relative humidity values. When the water content increased, electron solvation dominated over attachment to water molecules. Electron solvation led to dihydrogen production, albeit to a lesser extent than quasi-free electrons. Our experiments demonstrated the spontaneous curvature-induced charge separation in these inorganic nanotubes, making them very interesting potential co-photocatalysts.
by Furong Wang, Uli Schmidhammer, Jean-Philippe Larbre, Zizhao Zong, Jean-Louis Marignier and Mehran Mostafavi
Picosecond pulse radiolysis measurements were performed in neat D2O and H2O in order to study the isotopic effect on the time-resolved yield of the hydrated electron and hydroxyl radical. First, the absorption band of the hydrated electron in D2O, eD2O−, is measured between 250 and 1500 nm. The molar absorption coefficient of the solvated electron spectrum in D2O was determined using the isosbestic point method by scavenging the solvated electron using methyl viologen. The amplitude and shape of the absorption spectrum of the hydrated electron in D2O are different from those previously reported in the literature. The maximum of the hydrated electron in the D2O absorption band is ca. 704 nm with a molar absorption coefficient of (22 900 ± 500) L mol−1 cm−1. Based on this extinction coefficient, the radiolytic yield of eD2O− just after the 7 ps electron pulse was determined to be (4.4 ± 0.2) × 10−7 mol J−1, which coincides with the one for eH2O− in H2O. The time-dependent radiolytic yield of eD2O− was determined from a few ps to 8 ns. To determine the OD˙ radical yield, the contribution of the solvated electron and of the transient species produced by the electron pulse in the windows of the fused silica optical cell was taken into account for the analysis of the transient absorption measurements at 260 nm. Therefore, an appropriate experimental methodology is used for measuring low absorbance at two different wavelengths in ps pulse radiolysis. The yield of the OD˙ radical just after the 7 ps electron pulse was found to be (5.0 ± 0.2) × 10−7 mol J−1. In the spurs of ionization, the decay rate of eD2O− is slower than eH2O−, whereas the decay rate of OD˙ is similar to the one of OH˙. Here, the established time-dependent yield of the solvated electron and the hydroxyl radical provide the foundation for improving the models used for spur reaction simulations in heavy water mainly for the chemistry of CANDU reactors.
The behavior of carbonates is critical for a detailed understanding of aging phenomena in Li-ion batteries. Here we study the first reaction stages of propylene carbonate (PC), a cyclical carbonate, by picosecond pulse radiolysis. An absorption band with a maximum around 1360 nm is observed at 20 ps after the electron pulse and is shifted to 1310 nm after 50 ps. This band presents the features of a solvated electron absorption band, the solvation lasting up to 50 ps. Surprisingly, in this polar solvent, the solvated electron follows an ultrafast decay and disappears with a half time of 360 ps. This is attributed to the formation of a radical anion PC–•. The yield of the solvated electron is low, suggesting that the radical anions are mainly directly produced from presolvated electrons. These results demonstrate that the initial electron transfers mechanisms are strongly different in linear compared with cyclical carbonates.