Photochemical and Photophysical Processes on the Surface of Wide Band Gap Insulator Particulates: Gas/Solid System Involving Scandia (Sc₂O₃) Particles

Abstract

Surface photochemical processes and the interconnected photophysical events taking place in the bulk and at the surface of a wide band gap insulator material, e.g. scandia (Sc 2 O 3 ), have been examined to ascertain the role of intrinsic defects (e.g. anion and cation vacancies) on surface processes such as photoadsorption of electron acceptor (O 2 ) and donor (H 2 and CO) molecules. The influence of surface processes on the formation and destruction of photoinduced defects (i.e. electron F-type color centers and hole V-type color centers) in scandia and the role that color defect centers play in the overall efficiency of surface reactions have been explored spectroscopically. Photoadsorption of gases on the scandia particle surface and the resulting effect on photocoloration and photobleaching have been used to identify the type of color centers produced on photoexcitation with UV light in the fundamental absorption edge (intrinsic) and in the extrinsic absorption region of powdered scandia specimens. Formation of color centers red-shifts the spectral limits of surface photoreactions. Formation of F and V color centers decreases the quantum yield of surface photochemical processes because of recombination of charge carriers through color centers, but increases the overall efficiency of these processes through photobleaching of the color centers on irradiation at appropriate UV wavelengths.

Photo‐induced processes in heterogeneous nanosystems. From photoexcitation to interfacial chemical transformations

“…Equation (16) reflects the competition between internal and external cycles of the relaxation in heterogeneous systems. Our conclusions from equations (11) to (16) can be illustrated by our recent experimental results [1,2] summarized in Figures 3 and 4 for the processes of formation of color centers during photoexcitation in the fundamental absorption band of solids in both solid/gas and solid/liquid heterogeneous systems.…”

“…First, their presence, together with surface states, causes extrinsic absorption of light at energies much lower than the bandgap of solids [1,2]. Depending on the type of defects and on the mechanism of their photoexcitation, extrinsic absorption also leads to photoinduced generation of both electrons and holes [1][2][3][4].…”

“…The kinetics of formation of color centers under steadystate irradiation are illustrated in Figure 4a for ZrO 2 and Figure 4b for Sc 2 O 3 systems. It is clear that after a period of growth, the concentration of the color centers reaches steady-state, which is determined by the competing processes of formation and decay of the color centers from recombination events (see stages (4a)-(4f)) as determined by equation (1).…”

“…As a result, the spectral limit of photochemical and photophysical processes is red-shifted in pre-photocolored solids compared to their initial states. Thus, photocoloration leads to the spectral sensitization of solids in heterogeneous systems [1,2,4,6].…”

Photophysical Processes Related to Photoadsorption and Photocatalysis on Wide Band Gap Solids: A Review

“…PSPS of oxygen for wide band gap oxides is induced at repeated irradiation in F-type color centers band, while PSPS of donor molecules occur under photoexcitation of colored oxides in V bands. [66]. Note, in the pilot studies [13] with primary excitation of oxides by mercury lamp at E(hν 1 ) ≤ 6.0 eV and with repeated irradiation in visible (E(hν 3 ) ≤ 3.1 eV or 2.5 eV), the PSPS of oxygen accepted as the test gas has not been detected: i) for relatively narrow band gap poorly photocolored oxides (ZnO, TiO 2 , In 2 O 3 ), ii) for oxides with E g > 6.0 eV (Al 2 O 3 , BeO, MgO) and iii) for oxides without F-type color center bands in spectral region E(hν) ≤ 3.0 eV such as Y 2 O 3 and Sc 2 O 3 .…”