10.1117/2.120173.006823 Improving solar hydrogen production with photonic band gap materials Hicham Idriss, Vedran Jovic, and Geoff Waterhouse New gold/titanium dioxide photonic band gap semiconductor materials with an inverse opal structure are promising photocatalysts for solar hydrogen production. Hydrogen production from renewable sources is the most promising method to secure energy for future generations. 1 One approach to hydrogen production is photocatalysis, which requires the interaction between sunlight (photons) and a semi- conducting material. 2 Accordingly, the semiconducting material needs to maximally absorb photons, have a high work function metal layer to act as a Schottky barrier 3 —creating a reservoir of electrons to reduce hydrogen cations to molecular hydrogen— and have, in some cases, another metal oxide layer for fast oxidation of oxygen anions to molecular oxygen. 4 Photoexcitation of a semiconductor material creates electrons and holes—charge carriers—that recombine on a timescale much faster than that required for the electron transfer reaction to occur. 5 That is, we need to increase the lifetime of the charge carriers to make use of them in catalysis. Hence, photonic band gap (PBG) materials 6 designed to have a periodic 3D crystal structure with unit cell dimensions close to the wavelength of the light 7 have a key role in photocatalysis. The design of semiconductor photocatalysts with electronic band gaps (EBGs) that coincide with their PBGs ‘theoretically’ suppresses electron/hole recombination, a result of the lack of corresponding optical energy levels. This feature would make them ideal for photocatalysis and, therefore, the combination of slow photons at the edge of the PBG (increas- ing light absorption) and the inhibition of spontaneous light emission at the desired wavelength is poised to significantly increase photocatalytic reaction rates. Here, we describe our recent work with gold/titanium dioxide (Au/TiO 2 ) PBG semi- conductor materials. 8, 9 Figure 1. Synthetic routes (top), transmission electron microscopy images (bottom left), and x-ray diffraction data (bottom right) of gold/titanium dioxide (Au/TiO 2 ) photonic band gap semi- conductor materials. CC: Colloidal crystals. HAuCl 4 .3H 2 O: Hydrated chloroauric acid. We designed and made TiO 2 PBG semiconductor materials with periodicities (i.e., unit cell repeat distances) Continued on next page