Hydrogen Evolution with Fluorescein-Sensitized Pt/SrTiO3 Nanocrystal Photocatalysts is Limited by Dye Adsorption and Regeneration

Abstract Photocatalysts are one of several technologies for the conversion of sunlight and water into hydrogen fuel. Here, we show that the photocatalytic activity of fluorescein-sensitized platinum modified SrTiO3 nanocubes for visible light hydrogen evolution from aqueous solutions of methanol, KI, hexacyanoferrate (II), and triethanolamine is limited by light absorption from the fluorescein and by its self-oxidation, even in the presence of the sacrificial donors. The highest hydrogen evolution rate was observed with Pt/SrTiO3 suspended in 0.5 mM fluorescein at pH 13 with triethanolamine as a sacrificial donor (60 μmol h-1, 0.67% AQE at 2.64 eV). The other donors (methanol and KI) gave lower H2 rates or no H2 (hexacyanoferrate (II)). This trend can be rationalized with surface photovoltage spectroscopy (SPS) results on FL-sensitized SrTiO3 films on indium tin oxide substrates. Electron transfer from the dye to the SrTiO3 under 2.75 eV illumination and hole transfer to the sacrificial donors is observed to be modulated by the redox potentials of the donors and acceptors. The low activity of hexacyanoferrate appears to be due to e/h recombination, while triethanolamine appears to replace the dye from the SrTiO3 nanocrystals under the conditions of the SPS experiment. Photovoltage values suggest charge transfer is accompanied by irreversible self-oxidation of FL and this is confirmed by transient surface photovoltage measurements. Overall these results suggest that system optimization requires dyes with greater redox stability and stronger binding affinity to the SrTiO3 nanocrystal surface.