The investigation of novel D-π-A type dyes (MK-3 and MK-4) for visible light driven photochemical hydrogen evolution

Two novel donor-π-acceptor (D-π-A) semiconductor organic dyes have been synthesized for the photochemical hydrogen evolution reaction (HER) to sensitize TiO2 for the first time. The molecular structures of D-π-A semiconductor organic dyes, which are entitled as MK-3 and MK-4, have been characterized by NMR spectroscopy method; and also electrochemical and optical properties have been investigated by cyclic voltammetry and UV–Vis absorption techniques, respectively. Amount of dye loading on TiO2 surface has been investigated by EDX method. The HER activities have been explored in the presence of triethanolamine (TEOA) as a electron donor reagent under sunlight (solar simulator illumination limited by cut-off filter λ ≥ 420 nm) in the absence and presence of co-catalysts (Pt and Cu2WS4). Transient photocurrent densities of MK-3/TiO2 and MK-4/TiO2 electrodes have been reached to 110 μA cm−1 and 275 μA cm−1, respectively. The photocatalytic HER activities have been relatively enhanced in the presence of the Pt or Cu2WS4 co-catalysts (dye/TiO2/Cu2WS4 or dye/TiO2/Pt) when compared to only dye/TiO2. The HER rates have been found as 427, 1277, 675, 682, 1027 and 795 μmolg−1h−1 for the MK-3/TiO2, MK-3/TiO2/Pt, MK-3/TiO2/Cu2WS4, MK-4/TiO2, MK-4/TiO2/Pt and MK-4/TiO2/Cu2WS4, respectively. The differences of photochemical activities of MK-3/TiO2 and MK-4/TiO2 have been stated taking into account dye molecule structures. Moreover, the HER mechanism have been described by using electrochemical band energy levels of dyes, TiO2 and Cu2WS4 co-catalyst.