Synthesis of organic materials from carbon dioxide and water using nitride semiconductor photo-electrode

Abstract

We report on an energy conversion system which converts CO₂ into organic energy source by light and water. For the photo-electrode, we applied the gallium nitride (GaN) with nickel oxide co-catalysts on its surface. It had been difficult to realize this reaction because the energy of excited electron is lower than that of CO₂ conversion in most oxide-based photo-catalysts; however, we firstly found that nitride semiconductor makes it possible to solve this problem and succeeded in realizing the CO₂ reduction by light illumination alone. The GaN photo-electrode has a hetero structure with Al-doped GaN (AlGaN) in order to take advantage of a polarization effect in AlGaN layer, which enhances the electron-hole separation with suppressing the recombination of them. By the design of thin film structure in nitride semiconductor and adoption of indium (In) cathode, the energy conversion efficiency from solar light to formic acid (HCOOH) reached 0.15 %, which is almost the same level as that of real plants. We can show a demonstration cell which enables us to show a chemical reaction by color change of test reagent. The outdoor experiment is also shown in which the photo current can be observed under the real sun light.