Abstract
H2-production from renewables using sunlight is probably the holy grail of modern science and technology. Among the many approaches for increasing reaction rates, by increasing light absorption, plasmonic materials are often invoked. Yet, most plasmonic metals on semiconductors are also good for Schottky barrier formation. In this work, we are presenting evidences of de-coupling the plasmonic from Schottky effects on photoreaction. To conduct this we have systematically changed the under-layer gold film thickness and associated particle size. On top of the thin film layer, we have deposited the exact amount of a prototypical Schottky-based photo-catalyst (Pd/TiO2). We found up to 4 times increase in the H2-production rate at a critical Au film thickness (8 nm-thick). Below this thickness, the plasmonic response is not too strong while above it, the PR decays in favor of the Drude absorption mode. The reaction requires the presence of both UV (to excite the semiconductor) and visible light (to excite Au particles) in order to obtain high hydrogen production, 800 µmol/gCatal.min (probably the highest direct hydrogen (not current) production rate reported on a performing catalyst). The enhancement origin is quantitatively traced to its computed electric field strength (EFS). Adding a dielectric (SiO2) in between the Au thin layer and the catalyst exponentially decreased the reaction rate and EFS, with increasing its thickness. This work indicates the possibility of making an active and stable photo-catalyst from fundamental concepts yet further progress on the structural (technological) front is needed to make a practical catalyst.Graphical abstract
Highlights
From the very first report of Fujishima and Honda, T iO2 has been widely regarded as the leading candidate for solar H2 production because of its excellent stability [1]
We have found a direct corelation between the trends of electric field (EF) enhancement of the Au films and the photocatalytic activity of the TiO2 coated on top
The Localized surface plasmons resonance (LSPR) for 2 nm Au film thickness is located around 570 nm, it is red shifted with increasing thickness up to 8 nm
Summary
From the very first report of Fujishima and Honda, T iO2 has been widely regarded as the leading candidate for solar H2 production because of its excellent stability [1]. There have been several studies reporting plasmonic enhanced photocatalytic activity of T iO2 by using Au and Ag nanoparticles [5,6,7,8]. These enhancements are either mild or could not be proven independently. In that regard, TiO2 has been studied in various forms such as nanoparticles, core shell structure and thin films with Au and Ag nanoparticles to unravel the major mechanisms involved in plasmonic photocatalysis and to provide physical explanations for enhanced activities [7, 9]. The mechanism by which plasmonic metal nanoparticles improve energy conversion maybe grouped in two categories as follows:[2, 3]
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