Relative photonic efficiencies and quantum yields in heterogeneous photocatalysis

Abstract

Quantum yield and quantum efficiency (QY) as used in heterogeneous photocatalysis (solid/liquid or solid/gas systems) have too often been used incorrectly to mean the ratio of the rate of a given event to the rate of incident photons impinging on the (external) rector walls, typically for broadband radiation. There is little accord on how to express process efficiency. At times QY is defined, often ill-defined, and more frequently workers fail to describe how it was assessed. This has led to much confusion in the literature, not only because of the different meaning of QY from that in homogeneous photochemistry, but also because this method of describing photon efficiency precludes a comparison of results from different laboratories because of the variations in light sources, reactor geometries, and overall experimental conditions. It cannot be overemphasized that the reported QY is an apparent quantum yield, indeed a lower limit of the true quantum yield. This position paper addresses this issue, and argues that any reference to quantum yields or quantum efficiencies in a heterogeneous medium is ill-advised unless the actual number of photons absorbed by the light harvester (the photocatalyst) has been determined. The extent of light scattering in a solid/liquid heterogeneous medium is significant. A practical and simple alternative for comparing process efficiencies was recently suggested by defining a relative photonic efficiency ζ_r. A quantum yield can subsequently be determined from ζ_r, as φ = ζφ_phenol, where φ_phenol is the quantum yield for the photocatalyzed oxidative disappearance of phenol (a standard secondary actinometer) using Degussa P-25 TiO₂ as the standard catalyst material.