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Preparation of Inorganic Photocatalytic Materials for Overall Water Splitting

Photocatalysis, using powder-form materials, is the catalytic reaction initiated by the excited charge carriers (electrons and holes) that are generated by the absorption of the photons in the semiconductor materials (photocatalysts). Both the generated electrons and the holes move to the surface of the photocatalytic materials and subsequently initiate their respective redox reactions. The photocatalytic process fully utilizes the electronic configuration occurring at the metal–semiconductor and semiconductor–electrolyte interfaces by constructing band structures (the formation of band bending and barriers) without introducing p–n junctions. This electronic structure is essential in enhancing the charge separation and in achieving a highly efficient photocatalytic process because the photophysical process, including the generation and recombination of the excited carriers, occurs on a significantly shorter time scale (femtoto-microseconds) than surface electrocatalytic reactions, unless the process is separated effectively (microseconds to several seconds). A high photon efficiency for photocatalysis can only be achieved when the charge recombination, which occurs in the bulk and surface of the photocatalyst and in the cocatalyst, is suppressed as much as possible. In the bulk of the photocatalysts, it is generally accepted that the defects that disturb the smooth charge transfer (e.g. , reduced metal sites, cation and anion vacancies, phase segregation, crystallographic shear) act as charge recombination sites. On the surface, electrons should selectively flow from the photocatalyst to the hydrogen evolution cocatalysts (generally metal nanoparticles) or from the oxygen evolution cocatalysts (generally metal oxide nanoparticles) to the photocatalyst after overcoming the (Schottky) barriers. The surface between the semiconductor and the cocatalyst particles should be designed to possess ohmic contact with the minimized barrier. The cocatalysts must be highly electrocatalytically responsive to the surface redox reactions. The cocatalysts should accommodate either electrons for hydrogen evolution or holes for oxygen evolution; otherwise, the cocatalysts circumvent the charges. A semiconductor that can be used as a photocatalyst for overall water splitting must satisfy the thermodynamic requirement that it possesses the suitable band positions relative to the water redox potentials. The bottom of the conduction band has to be positioned at a greater negative potential than the water reduction potential, and the top of the valence band has to be located at a more positive potential than the water oxidation potential. The design of the photocatalysts should be focused on the following points: