Efficient artificial photosynthesis, in which energy from sunlight is stored in chemical bonds, has remained an elusive goal. The photoelectrolysis of water into H2 and O2 is a key component of this project. It was first achieved by Fujishima and Honda using an n-type TiO2 electrode in 1972, but efforts to develop a more efficient process have generally faltered. Materials whose band gaps are well matched to the solar spectrum are chemically unstable in water and therefore unsuitable for photoelectrochemical cells. TiO2 has a 3 eV band gap and can only absorb ultraviolet light (400 nm or shorter), setting its theoretical maximum energy conversion efficiency at 2.2%. To date, however, TiO2-based cells make insufficient use of even the UV spectrum, with incident photon-to- current efficiencies (IPCE) of 10% or less at the band gap and peak energy conversion efficiencies of 0.6% or less over the whole solar spectrum. A primary factor that limits the efficiency of these films is the competition between the optical path length required for light absorption and charge diffusion lengths. It is thus important to engineer systems with both high optical density and high surface area-to-volume ratio.
展开▼
