A strategy of engineering impurity distribution in metal oxide nanostructures for photoelectrochemical water splitting

Impurity doping has been evidenced as one of the most effective methods to activate wide band gap semiconductors in visible light. However, these doped metal oxides always encounter serious charge recombination at the impurity levels introduced by the foreign dopants, leading to relatively low solar water splitting performances. In this perspective, a strategy of engineering impurity distribution is presented for metal oxide nanostructures for improved photoelectrochemical water splitting under visible light. Particular attention is paid to those doped systems with optical absorption and electron transport decoupled by spatially engineering the associated impurity distribution. In the context of this discussion, some selected systems of inhomogeneously doped ZnO nanostructured photoelectrodes, in which the visible-light absorbing and charge conducting regions are of isostructural nature to avoid introducting large amounts of interface recombination centers, are briefly discussed. Later on, a concept of impurity distributed homojunction is demonstrated to enhance charge separation by implementing a gradient in the dopant concentration. An ion implantation method is used to inject foreign dopants into ZnO and TiO2 nanorod arrays to create an impurity distributed homojunction with enhanced optical absorption and photoelectrochemical water splitting performances in visible light. The electronic and physicochemical properties of these ion implanted samples are of great dependence on the implanted ions as well as the metal oxide substrates. In particular, the N ion implanted ZnO nanorod arrays show good photoelectrochemical performance in visible light, because the gradient distribution of N dopants in ZnO nanorods not only extends optical absorption to visible light region, but also introduces internal driving force for charge carrier separation by the gradient band bending in the N dopants distributed ZnO homojunction. It is then conclusive that by spatially engineering impurity distribution, metal oxide nanostructures could be activated in visible light, synergistically with promoted charge carrier separation, for enhanced photoelectrochemical water splitting performances in visible light.