Probing the Role of Surface Energetics of Electrons and their Accumulation in Photoreduction Processes on TiO₂

Abstract

We address the role of the energetics of photogenerated electrons in the reduction of 4-nitrobenzaldehyde on TiO2. This model molecule bears two functional groups featuring different reducibilities. Electrochemistry shows that reduction to 4-aminobenzyl alcohol occurs in entirely distinct potential ranges. Partial reduction of the -NO2 group, affording 4-aminobenzaldehyde, takes place through surface states at potentials positive of the flatband potential (E(fb)). Dark currents caused by reduction of the aldehyde group are observed only at potentials more negative than E(fb), and the process requires an electron accumulation regime. Photocatalysis with TiO2 suspensions agrees with the electrochemical data. In particular, reduction of the nitro group is a relatively fast process (k=0.059 s(-1)), whereas that of the aldehyde group is slower (k=0.001 s(-1)) and requires electron photoaccumulation. Control of the photogenerated charge is a prospective means for achieving chemoselective reductions.

Charge carrier dynamics and visible light photocatalysis in vanadium-doped TiO2 nanoparticles

“…This result is an indication that this photocatalyst is able to reduce the nitro group using photoexcited electrons from visible absorbing states. From a mechanistic point of view, it has been demonstrated that the protons required to form the NH2 group are provided by the dissociative adsorption of sacrificial C3H8O (isopropanol, 2-Pr-OH) on the NPs surface and its subsequent oxidation into C3H6O (acetone) [52]. Further details on the mechanism of H + formation are provided in the Supplementary Material.…”

“…If the spectral range of the incident light is extended in the UV, e.g. by using a  > 360 nm filter, the photocatalytic reduction takes place in few minutes [52] and a significant difference between undoped and V-doped TiO2 NPs can no longer be detected.…”

Reactivity of Trapped and Accumulated Electrons in Titanium Dioxide Photocatalysis

“…Molinari and coworkers examined the selective hydrogenation of NO 2 -C 6 H 4 -CHO, bearing the two reducible functional groups, -NO 2 and -CHO [62]. They found that the nitro group was easily reduced by the trapped electrons at Ti ds within the bandgap, whereas the aldehyde group was reduced by electrons accumulated on CB.…”

“…They also revealed that the total density of the traps is well-correlated to the specific surface area of TiO2 powders, suggesting that the electron trap sites are predominantly located on the surface of TiO2, and they do not depend on the type of crystallites in anatase, rutile, or their mixtures. The energy distribution of Ti ds can also be obtained by an electrochemical method [61,62]. In this method, the potential variation in accumulated charge at the TiO 2 electrode can be measured in aqueous solution.…”

“…Notably, the relative position between the Ti ds energy distributed within the bandgap and the acceptor level of the AP derivatives (the solid Gaussian curve) should be appropriate ( Figure 5) [72]. The energy distribution of Ti ds in TiO 2 powders and colloids can be obtained by photochemical [60], electrochemical [61,62], and RDB-PAS [56] methods. The acceptor levels of AP derivatives can be estimated by the Marcus theory [72,[74][75][76].…”