Mechanisms of Photocatalytic Molecular Hydrogen and Molecular Oxygen Evolution over La-Doped NaTaO3 Particles: Effect of Different Cocatalysts and Their Specific Activity

A better understanding of the mechanisms of H2 and O2 evolution over cocatalyst-loaded photocatalysts is an essential step in constructing efficient artificial systems for the overall water splitting. In this paper, La-doped NaTaO3 particles loaded with different cocatalysts (i.e., noble metals and metal oxides) have been synthesized and used as model photocatalysts to study the mechanisms of photocatalytic H2 and/or O2 evolution from pure water, aqueous methanol solution, and aqueous silver nitrate solution. It was found that the photocatalytic activity and selectivity toward H2 and/or O2 evolution strongly depend on the nature of the cocatalyst and the investigated system. For pure water and aqueous silver nitrate systems, the affinity of the cocatalyst nanoparticles to react with the photogenerated charge carriers (electrons or holes) was found to be the main reason for the observed selective behavior for H2 and O2 evolution. The creation of active sites and subsequent decrease in activation energy is thought to play a secondary role. In the presence of methanol, when the dark injection of an electron into the conduction band of the photocatalyst is possible, the catalytic roles of the investigated cocatalysts toward the formation of H2 gas were found to be decisive, in addition to the charge separation and interfacial electron transfer processes. No overall water splitting into H2 and O2 can be achieved utilizing La-doped NaTaO3 loaded with only one cocatalyst; however, it was found that the loading of La-doped NaTaO3 with two different cocatalysts, i.e. RuO2 and CoO, enables the simultaneous formation of H2 and O2 from pure water. The modification of photocatalyst with two different cocatalysts seems to be essential for enhancing the efficiency of overall photocatalytic splitting. The interfacial electron transfer on the cocatalyst-loaded La-doped NaTaO3 was determined by measuring the cathodic and anodic photocurrents in the presence of Fe2+/Fe3+ electron shuttle. Methanol and bromate were used as electron donors and electron acceptors during the cathodic and anodic photocurrent measurements, respectively. By correlation of the photocurrent with the activity of the investigated cocatalysts, it was concluded that the creation of active sites and subsequent decrease in activation energy for H2 evolution is the main requirement for efficient H2 evolution from the aqueous methanol system, whereas the interaction with the photogenerated holes and the formation of intermediates allowing a multielectron transfer process seems to be an essential step for the water oxidation and O2 evolution. This information appears to be crucial for a rational design of a highly active photocatalyst for overall water splitting under UV–vis illumination.