Enhanced hydrogen production from ammonia borane using controlled plasmonic performance of Au nanoparticles deposited on TiO2†
Ammonia borane (AB) is a hydrogen storage material which can produce three equivalent moles of H2 gas by a hydrolytic decomposition reaction in the presence of an appropriate catalyst. The photocatalytic H2 generation from AB decomposition was studied by utilizing the Au/TiO2 plasmonic photocatalyst under both visible and UV light conditions. Different amounts of gold (0.5, 1, 2, and 3%) were photo-deposited on TiO2 (anatase) to prepare Au/TiO2 catalysts. Au(1 wt%)/TiO2 showed the highest activity of H2 generation under visible light and a further increase of Au loading reduced the activity. Although LSPR (localized surface plasmon resonance) excitation should increase with gold nanoparticle (Au NP) loading, excessive Au loading also facilitates the recombination of LSPR charge pairs. The time-resolved photoluminescence (TRPL) decay spectra of Au/TiO2 clearly showed that the recombination rate of LSPR charge pairs is accelerated at higher Au loading. As a result, the photocatalytic activity of Au/TiO2 was optimized at 1 wt% Au loading to produce 88 μmol of H2 in 4 h. However, such an optimized level of Au loading was not observed under UV light irradiation conditions and the UV photocatalytic production of H2 increased with the Au loading up to 3% because higher Au loading facilitates the separation of charge pairs and accumulation of electrons in Au NPs through the Schottky barrier. These two different electron transfer behaviors (accumulation and recombination) can be further confirmed by the Cr(VI) reduction experiments since they showed a similar trend of photocatalytic activity with Au loading. This research provides a fundamental understanding for optimizing the photocatalytic activity of various metal–metal oxide heterojunction photocatalysts.