Spectral Dependencies of the Quantum Yield of Photochemical Processes on the Surface of Nano/Micro-Particulates of Wide-Band-Gap Metal Oxides. IV. Theoretical Modeling of the Activity and Selectivity of Semiconductor Photocatalysts with Inclusion of a Subsurface Electric Field in the Space Charge Region

Abstract

In an earlier study (Emeline, A. V.; Ryabchuk, V. K.; Serpone, N. J. Phys. Chem. B 1999, 103, 1316) we reported the solution to the continuity equation in which we neglected electric field effects and obtained expressions for the concentrations of the charge carriers at the surface (n s), for the quantum yield (Φ) of a photochemical surface redox reaction, together with an expression for the selectivity of the photocatalyst. Various considerations led us to infer and predict wavelength-dependent phenomena (quantum yields and selectivity), which were later confirmed by experimental results (Emeline, A. V.; Serpone, N. J. Phys. Chem. B 2002, 106, 12221). In this article we revisit the continuity equation but include a subsurface electric field (E) in the space charge region of the semiconductor (or dielectric) photocatalyst to assess the effect that the electric field has on the activity and selectivity of the photocatalyst. The solution to the continuity equation yields very complex expressions for n s and for Φ. Nonetheless, several cases are considered which simplify the expressions. Taking E = 0, the expression for the quantum yield transforms into the identical equation obtained earlier. Some of the predictions from the expressions obtained herein also find experimental verification from considerations, for example, of the quantum yields of photoadsorption of dioxygen and dihydrogen on TiO2 particles. The agreement between the predicted spectral behavior of the ratio γ = [es]/[hs] against wavelength with the spectral dependence of ΦO 2 /ΦH 2 is remarkable. Another no less significant conclusion from the theoretical modeling results is that a mismatch between the experimental spectral dependence of the quantum yield and the corresponding absorption band may be an indicator of the existence of a space charge region in the solid photocatalyst.

A surface science perspective on TiO2 photocatalysis

“…The high dielectric constant of TiO 2 greatly assists in separation as point charges moving away from each other become screened from their counter charges by the lattice. An imposed electric field [388,389] can also assist in charge separation, just as establishment of a space charge region can.…”

“…Conversely, trapping of a charge carrier at the surface (e.g., an electron at a surface Ti 4+ site) can provide the setting for formation of a stronger chemisorption interaction between a molecule and the surface (i.e., photoadsorption). Literature examples of photoadsorption on TiO 2 include O 2 , H 2 , H 2 O, NO, CH 4 and organic acids [9,387,388,556,778,[785][786][787][788][789][790]. Research is needed into what charge carriers and what surface sites are involved in these photoabsorption and photodesorption events, and how these events promote, regulate or inhibit photocatalytic processes on TiO 2 .…”

Heterogeneous photocatalysis: Light-matter interaction and chemical effects in quantum efficiency calculations

“…applicable to most common rector configurations) formulations to obtain quantum efficiency values [5][6][7][8][9][12][13][14][41][42][43][44][45]. Among light-matter and chemical parameters or effects influencing efficiency calculations, the most salient studies focussed on the following: (i) obtaining accurate measurements and modeling, with explicit consideration of all optical events, of the photon handling capability of photoactive catalytic surfaces or bodies [6,9,14,42,[44][45][46]; or (ii) accounting for the influence of the selectivity in the spectral dependence of the quantum efficiency [47]. The effect of catalyst morphology in the light-matter interaction as well as a detailed analysis of selectivity in the number of charge carrier species used in photocatalytic reactions was however not accounted for in the previous contributions.…”

Green oxidations: Titanium dioxide induced tandem oxidation coupling reactions

“…The positive holes and electrons migrate to the surface where the holes react with water (or bound hydroxyl groups) to produce hydroxyl radicals which are strong oxidants [21]. Nevertheless, the activation of titanium dioxide under microwave irradiation is surprising since UV light is more intense than microwave energy.…”