Toward a Physically Sound Structure-Activity Relationship of TiO₂-Based Photocatalysts

The morphological (analyzed by N2-Brunauer−Emmett−Teller, transmission electron microscopy, X-ray diffraction), surface chemical (analyzed by NH3-temperature programmed desorption), and opto-electronic properties (analyzed by time resolved microwave conductivity measurements (TRMC)) of nanoparticulate anatase TiO₂ (Hombikat, Sachtleben) were modified by calcination at temperatures ranging from 200 to 800 °C. Upon calcination the primary particle size and the crystallinity increase, strongly influencing the performance in the degradation of methylene blue. It is striking that for the investigated set of samples the rate versus primary particle diameter goes through a minimum. This is explained by the equation of the kinetic rate constant, keff = k[OH][h+]; [OH] decreases continuously as a function of increasing particle size, which is overcompensated by a strong increase in [h+] for particle sizes above ∼15 nm. The latter is the result of an increasing crystal quality of the anatase phase induced by calcination, as derived from the TRMC measurements. Implications for the rational design of improved photocatalysts are discussed