(Invited) Photon-Density Dependence of Photocatalysis: Can Standard Electrode Potential be Applied to Particulate Photocatalysis?
Authors: Ohtani Bunsho , Takeuchi Shugo , Takashima Mai , Takase Mai 
Published: February 2020

For heterogeneous photocatalysis, it is well known that photocatalytic oxidation of water into oxygen (O2) is induced by particulate photocatalysts in the presence of electron acceptors. This reaction has been presumed to proceed through a four-electron (hole) process, and it is thereby expected that reaction rate and efficiency depend on irradiation-light intensity and particle size of a photocatalyst, which influence the number of photons absorbed by one photocatalyst particle. In general, for the improvement of reaction efficiency, the reaction of electron-hole pairs such as enhancing charge separation and decreasing recombination has been focused, but photoabsorption, the first step of photocatalytic reaction, has been negligibly discussed so far. In this study, we analyzed light-intensity and particle–size dependences of rate of photocatalytic O2 evolution using commercial titania powders in the presence of iodate (IO3 - ) or iron(III) (Fe3+) ion as an electron acceptor for clarification of correlation of reaction rate with photoabsorption.
Photoirradiation on titania suspensions (30 mg in 3.0-mL deaerated aqueous electron-acceptor solutions in a quartz cell) was performed using high light-intensity UV-LEDs (365 nm; NS Lighting ULEDN-101 and Hamamatsu Photonics L11921-400) and an aperture (1.0 cm square) with overall intensity up to ca. 340 mW.
The observed light-intensity dependence of rate with small-size (4.4 nm) anatase titania as a photocatalyst using IO3 - as an electron acceptor, in the relatively lower intensity region, was second (< 100 mW) and first-order (< 280 mW) as was also seen in the reactions with small-size (13 nm) rutile titania even in the cases in which Fe3+ or IO3 - was used as an electron acceptor. This dependence is attributable to two-electron process (to yield hydrogen peroxide) and the linear (first-order) dependence at the middle-intensity region might be due to the sufficiently high intensity to guarantee the two-photon absorption by one photocatalyst particle. At the further higher intensity (> ca. 280 mW), on the other hand, fourth-order dependence appeared. Since the electrode potential of four-electron reaction (+1.23 V) is lower than two-electron reaction (+1.77 V), it was presumed that the probability of four positive-hole accumulation in one particle under high-intensity irradiation became appreciable to induce four-electron transfer to water. On the other hand, large-size (ca. 350 nm) rutile titania showed only first-order light-intensity dependence only in the whole range of light intensity, suggesting sufficient accumulation of positive holes in the particles. These results suggest that multi-electron reaction mechanism depends on the density of electron (hole) in one photocatalyst particle, i.e., the rate is digitally controlled when size of photocatalyst particles is small [1]. It is, therefore, expected that the efficiency of photocatalytic multielectron-transfer reactions can be improved so much by only focusing the irradiation beam without changing the overall intensity. At the same time, it is suggested that for heterogenous photocatalysis using particulate semiconductors it is not straightforword to predict the possiblity of reaction on the basis of standard electrode potentials and band position assuming the guaranteed multielectron transfer.
[1] S. Takeuchi, M. Takashima, M. Takase and B. Ohtani, in preparation.