Macroporous Fluorine Doped Tin Oxide Photoelectrodes for Solar Water Splitting

Fluorine doped tin oxide (FTO) is widely used as a back contact transparent conducting oxide (TCO) in photoelectrodes for solar energy applications. Using relatively complex methods, a variety of materials can be deposited over FTO to form porous electrodes, which are desired for their higher surface areas. The present work shows how it is conceptually much simpler to template the TCO electrode, producing macroporous electrodes, which allows relatively simple methods to be used to deposit secondary materials for solar water splitting.
Macroporous FTO electrodes (macFTO) were fabricated to give highly conductive and structurally ordered electrodes. The fluorine content was analysed using solid state NMR and was found to be as low as 0.5 atom%, 100 times smaller than the precursor Sn:F ratio. The capacitance of electrodes was used to determine the surface area enhancement of macFTO compared to planar FTO (pFTO), which was found to be 12 times greater.
As macFTO is an ordered porous, or photonic, electrode, there is a possibility that an increase in light absorption efficiency could be observed. An investigation found that the stop band of macFTO was angular dependent, and the electrode could enhance the lifetime of embedded dyes. The emission lifetime of embedded Rudcbpy, a ruthenium based dye, was found to increase by a factor of 1.8, while the lifetime of coumarin 440/460 was increased by a factor of 1.4 when the stop band overlapped the emission profile of the dye, consistent with literature.
Finally, visible light absorbing materials and catalysts for water oxidation were deposited over macFTO, showing that simple techniques could be used to deposit a variety of useful materials. Independently, both BiVO4 and CdS nanoparticles were successfully deposited over macFTO. Only a low CdS nanoparticle loading was required to show that the macFTO electrodes were conductive and capable of supporting photocurrents, with up to a 100 fold increase in the photocurrent density observed compared to equivalent planar electrodes due to the surface area enhancement. The activity of macFTO-BiVO4 electrodes towards water oxidation was then tested with the use of a co-catalyst (CoPi). It was found that the activity of the macroporous electrode was substantially better than equivalent planar electrode, with oxygen evolution rates of 74 μL hr-1 produced with a bias of 1.6 VRHE, and faradaic efficiencies for oxygen evolution reaching 100%.