Colloidal metal oxide particles loaded with synthetic catalysts for solar H2 production.

This discussion describes a study of a particulate semiconductor assembly consisting of a photoactive ruthenium dye-sensitised metal oxide nanoparticle loaded with a synthetic cobaloxime proton reduction catalyst (CoP). The colloidal system evolves H2 during visible light illumination in pH neutral aqueous solution in the presence of a sacrificial electron donor. The ruthenium photosensitiser (RuP) to cobaloxime loading ratio is simple to vary and optimise and a range of metal oxide nanoparticles were tested. A maximum photoactivity was identified with TiO2 nanoparticles modified with CoP and RuP in a 2: 1 ratio in the colloidal reaction mixture: visible light irradiation yielded 600 +/- 32 micromol H2 h(-1) (g TiO2)(-1). A total turnover number of 108 +/- 9 mol H2 (mol CoP)(-1), the evolution of 4340 +/- 240 micromol H2 (g TiO2)(-1) and approximately 87 micromol H2 (m2 TiO2)(-1) were observed after 10 h irradiation. Linkage of the catalysts to TiO2 is critical for the system to work efficiently, and CoP and RuP contain one and two phosphonic acid linker moieties, respectively. The novel phosphonate ester analogue of CoP, [CoCl(dimethylglyoximato)2(diethyl pyridyl-4-phosphonate)] (1) was also synthesised and studied. Complex 1 adsorbs only to a small extent to TiO2 and a reduced H2 production rate (182 +/- 8 micromol H2 h(-1) (g TiO2)(-1)) was observed when 1 was irradiated with RuP-modified TiO2. Thus, the lower TiO2 affinity of 1 results in a reduced photoactivity of the dispersion. The described semiconductor particles are also presented in the light of being advantageous over more established homogenous multi-component systems and supramolecular dyad complexes: the reported semi-heterogeneous system is straightforward to assemble and it works in a purely aqueous environment.