Enhancing Charge Transfer in Photocatalytic Hydrogen Production over Dye-Sensitized Pt/TiO2 by Ionic Liquid Coating

Photocatalytic hydrogen production from water is considered to be a potentially cost-efficient method to produce hydrogen fuel with little impact on the environment. Nevertheless, hydrogen production efficiencies via photocatalysis remain to be low. Here, a photocatalyst system composed of 1 wt % Pt/TiO2 and dye-sensitized Pt/TiO2 particulates encapsulated by an ionic liquid is proposed. In particular, the enhancement of photocatalytic hydrogen production over 1 wt % Pt/TiO2 particulates sensitized with N719 dye (di-tetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylato) ruthenium(II)) and coated by a thin layer of ionic liquid [BMIM][BF4] (1-butyl-3-methylimidazolium tetrafluoroborate) has been investigated. The Pt was loaded onto the TiO2 surface by incipient wetness impregnation, while the dye and ionic liquid were loaded by solvent evaporation using ethanol and acetone, respectively. The photocatalytic tests were performed in a semibatch glass reactor under visible light irradiation provided by a solar simulator; the hydrogen production rates over Pt/TiO2, dye-sensitized Pt/TiO2, ionic liquid-coated Pt/TiO2, and ionic liquid-coated Pt/TiO2 after dye sensitization were compared. The hydrogen production rate was ∼21 μmol/h·gcat on Pt/TiO2, and it increased to ∼27 μmol/h·gcat with dye sensitization, while the [BMIM][BF4] coating alone improved hydrogen production three times (∼60 μmol/h·gcat). Coating the dye-sensitized particulates with an ionic liquid resulted in another 17% increase in hydrogen production (∼70 μmol/h·gcat). Scanning electron microscopy–energy dispersive X-ray analysis (SEM–EDAX), contrast transmission electron microscopy (CTEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, UV–vis characterization of the photocatalysts and electrochemical analysis of the respective photoelectrodes were also performed. The ionic liquid coating increased the performance of Pt/TiO2 by providing better charge transfer between the photocatalyst and the aqueous reaction medium while simultaneously preventing the recombination of photogenerated electron–hole pairs. The improved performance of the dye-sensitized photocatalyst upon the ionic liquid coating indicated that the ionic liquid stabilized the dye on the photocatalyst surface while simultaneously enhancing the charge transfer between the dye and TiO2.