Metal phosphates for the design of advanced heterogeneous photocatalysts

Research in heterogeneous photocatalysis is burgeoning and rapidly evolving given the benefits of performing demanding chemical transformations under mild conditions. Remarkable success has been achieved in photodegradation as an advanced oxidation option in water or air decontamination. Artificial photosynthesis, whereby photonic (sunlight) energy can be stored in the form of (solar) fuels, represents an encouraging strategic goal. Specially in the latter case, the design of effective light-absorbing semiconductors with properly aligned band potentials and co-catalysts enabling intricate multi-electron redox reactions is key. As described in this review, metal phosphates offer distinct possibilities to become tuneable semiconductors with regards to the traditional oxides, owing to the effects of introducing phosphorus in electronic and lattice structures. This is most clearly exemplified in the case of Ag3PO4, slightly widening the bandgap of the parent oxide while maintaining appropriate valence band energies, which in conjunction result in highly active oxidation activity under visible light. Other non-noble late (e.g. Co, Cu, Fe, Ni) or early (e.g. Ti, Zr, V, Nb) transition metal phosphates tend to have superior reducing properties, and band engineering might result in efficient photocatalysis for fuel production, chiefly H2 evolution and CO2 reduction. Their more complex structural features require dedicated synthetic efforts, regrettably overlooked hitherto, to effectively explore their real possibilities. BiPO4 is a UV-active semiconductor with ample applicability in photocatalysis, including hydrogen production; band engineering and other approaches to enable its response to visible light are discussed. Natural phosphates or hydroxyapatites are proven as resourceful families of structural or light-responsive components via band engineering by foreign metal doping. Beyond semiconductors, the discovery and development of surface metal (mostly cobalt) phosphates as highly efficient O2 evolution co-catalysts have been key to the advancement of solar water splitting. This review scrutinises and thoroughly analyses research activity on the use of metal phosphates as key components in advanced photocatalysis, including heterojunction nanocomposites and photoelectrochemical systems with other semiconductors and catalysts.