This review article revisits the band model often used to describe semiconductors following recent findings from our laboratory that such a model is inadequate to explain the generally occurring spectral dependence of quantum yields (φ) of surface photocatalyzed reactions by irradiated metal-oxide nanomaterials in several gas/solid and liquid/solid heterogeneous systems. A one-dimensional model is described to account for experimentally observed spectral variations of φ. From the solution to the continuity equation and given boundary conditions, certain predictions are made that are consistent with and can explain currently available experimental data. Some key factors that affect the activity and selectivity of photocatalysts in heterogeneous photocatalysis are identified and their relevance discussed; for example, surface charge (i.e. surface potential) and the subsurface electric field in the space charge region are two such key factors.