Nanoplasmonic Enhancement of Photon-to-Electron Conversion in Functionalized Iron Oxide Photoanodes

Licentiate thesis, 2012

The ever increasing energy demand of modern society clashes with the accelerating depletion of the fossil fuels from which most of this energy is taken today. Moreover, fossil fuels consumption poses a severe threat on the Earth’s environment and climate through global warming. The only option to solve our energy and environmental challenges simultaneously is a transition to a society based on sustainable energy. One appealing possibility is to harvest solar energy and store it in form of hydrogen molecules produced from water splitting in a photoelectrochemical (PEC) cell. In order to make PEC water splitting competitive for large-scale hydrogen production, several issues regarding device efficiency, stability and production cost need to be tackled. Hematite (Fe2O3) is a promising material for PEC cells despite the low values of energy conversion efficiency reported so far, which are partly due to a substantial difference in the characteristic lengths for light absorption and charge carrier transport. In this work, the influence of functionalization with Au nanoparticles on the PEC properties of hematite photoanodes was explored. Metallic nanoparticles support localized surface plasmon resonances, i.e., collective, non-propagating oscillations of electrons. When such resonances are excited, the electric field distribution around the nanoparticles is strongly altered leading to an enhanced charge generation and/or separation in hematite if the nanoparticles are in close vicinity to/in contact with it. In order to investigate and understand how Au nanoparticles affect the PEC performance of hematite, model photoanodes were fabricated and characterized. The functionalized photoanodes showed considerably higher photocurrent than the as-fabricated samples. Wavelength resolved photocurrent measurements revealed a clear increase in internal photon-to-electron conversion efficiency, providing the nanoplasmonic resonance was matched in energy to the bandgap in hematite. The origin of such increase was elucidated by comparison with optical extinction spectra and with calculated maps of field enhancement around the Au nanodisks. The improvement was attributed to enhanced charge generation close to the hematite-electrolyte interface caused by the electric field enhancement in hematite. The results presented here are applicable to photoanodes based on semiconductors with similar properties to hematite and are believed to be helpful in future design of photoanodes, where functionalization with metallic nanoparticles is combined with other strategies to improve the energy conversion efficiency, for instance material doping and nanostructuring.