Low-Temperature Atomic Layer Deposition of Crystalline and Photoactive Ultrathin Hematite Films for Solar Water Splitting.

We developed a low-temperature atomic layer deposition route to deposit phase pure and crystalline hematite (α-Fe2O3) films at 230 °C without the need for postannealing. Homogenous and conformal deposition with good aspect ratio coverage was demonstrated on a nanostructured substrate and analyzed by transmission electron microscopy. These as-deposited α-Fe2O3 films were investigated as photoanodes for photoelectrochemical water oxidation and found to be highly photoactive. Combined with a TiO2 underlayer and a low-cost Ni(OH)2 catalyst, hematite films of less than 10 nm in thickness reached photocurrent densities of 0.3 mA cm(-2) at 1.23 V vs RHE and a photocurrent onset potential of less than 0.9 V vs RHE, previously unseen for films this thin and without high temperature annealing. In a thickness-dependent photoelectrochemical analysis, we identified a hematite thickness of only 10 nm to yield the highest internal quantum efficiency when using a suitable underlayer such as TiO2 that induces doping of the hematite film and reduces electron/hole recombination at the back contact. We find that, at high bias potentials, photocurrent density and quantum efficiency proportionally increase with light absorption in films thinner than 10 nm and are limited by the space charge layer width in thicker films. Thus, we propose to apply hematite films of 10 nm in thickness for future developments on suitable nanostructured conductive scaffolds that can now be extended to organic scaffolds due to our low-temperature process.