Solar splitting of water to hydrogen and oxygen is a critical chemical transformation for which no commercially viable photocatalytic systems exist. Developing materials that could execute this reaction with high efficiencies would fundamentally change the environmental footprint of many segments of our economy, including chemical industry, manufacturing and energy sectors. The materials that have received the most attention for this application are hybrids that contain a semiconductor light absorber and an attached metal electrocatalyst that performs chemical transformations. In these systems, the semiconductor serves to provide the electromotive force (voltage) that is used by the electrocatalysts to drive the reaction. The main problems in the development of these hybrid photocatalysts are the chemical instability of the desired semiconductors (with the appropriate, relatively low band gap) under the relevant water splitting conditions, and the losses associated with the presence of the semiconductor/electrocatalyst junction in these photo-catalytic system.

Recently, it has been demonstrated that these low band gap semiconductor light absorbers can in some cases be stabilized by the use of protective insulator layers by forming metal/insulator/semiconductor (MIS) photocatalysts. This strategy incorporates a stable insulator layer placed between a metal electrocatalyst and semiconductor forming a metal-insulator-semiconductor junction. So far the central focus of this area of research has been on experimentally demonstrating the enhanced stability of the semiconductors covered by these insulating layers under the reaction conditions. In this contribution, we will discuss the realistic targets for solar water splitting for these MIS photocatalyst. We will also analyze the critical problems associated with these multi-component photocatalysts. We will discuss the impact of various components on the photo-catalysts performance. We will also show how by controlling the geometries of these systems at atomistic level we can optimize their solar to hydrogen efficiencies.