Dye-sensitized SrTiO3-based photocatalysts for highly efficient photocatalytic hydrogen evolution under visible light

Photocatalytic hydrogen evolution over cyanine-sensitized Ag/TiO2

Sensitization of TiO₂ by dyes such as cyanine and their derivatives is used as a technique to improve potency for the production of hydrogen gas as an alternative green fuel. These dyes shift the spectrum of TiO₂ from the UV region to the visible region, enabling it to harvest as much sunlight as possible. Herein, four different derivatives of cyanine (labelled C1, C2, C3, and C4) were prepared and doped in Ag/TiO₂via the impregnation method. The properties of the prepared photocatalysts were studied by XRD, SEM-EDS, FTIR, and UV-visible spectroscopy. The sensitized photocatalysts exhibited a similar morphology, nanoscale particle size, and good absorbance in the visible region. The rate constant for the photocatalytic activity of Ag/TiO₂ showed a great enhancement for hydrogen evolution after sensitization from 0.088 to 0.33 μmol min⁻¹. Doping of the C2 derivative in Ag/TiO₂ promoted the photocatalytic and sonophotocatalytic rates of H₂ production by 7.5 and 9 times, respectively. Also, the amount of photocatalyst had a significant effect on the photocatalytic activity of the sensitized Ag/TiO₂, where 0.14 g was the optimum dose, giving the maximum yield at both the initial rate and 300 min. One of the important factors causing the efficiency to reach high levels is the inhibition of photogenerated electron/hole recombination. This was achieved by adding a small quantity of methanol, which increased the rate by 9 times. The stability of the prepared photocatalysts was tested, which gave good results even after their 5th use. All the results confirmed that the sensitization of metal oxides is a promising solution in industry to produce clean energy (H₂) in high quantities over highly stable photocatalysts.

Comparison between photo- and sonophotocatalytic hydrogen production.

Composite photocatalysts based on Cd1−xZnxS and TiO2 for hydrogen production under visible light: effect of platinum co-catalyst location

Ternary composite photocatalysts based on titania and solid solutions of CdS and ZnS were prepared and studied by a set of physicochemical methods including XRD, XPS, HRTEM, UV-vis spectroscopy, and electrochemical tests. Two synthetic techniques of platinization of Cd_1−xZn_xS/TiO₂ were compared. In the first case, platinum was deposited on the surface of synthesized Cd_1−xZn_xS (x = 0.2–0.3)/TiO₂ P25; in the second one, Cd_1−xZn_xS (x = 0.2–0.3) was deposited on the surface of Pt/TiO₂ P25. The photocatalytic properties of the obtained samples were compared in the hydrogen evolution from TEOA aqueous solution under visible light (λ = 425 nm). The Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/Pt (1 wt%)/TiO₂ photocatalysts demonstrated much higher photocatalytic activity than the Pt (1 wt%)/Cd_1−xZn_xS (10–50 wt%; x = 0.2–0.3)/TiO₂ ones. It turned out that the arrangement of platinum nanoparticles precisely on the titanium dioxide surface in a composite photocatalyst makes it possible to achieve efficient charge separation according to the type II heterojunctions and, accordingly, a high rate of hydrogen formation. The highest photocatalytic activity was demonstrated by 20% Cd_0.8Zn_0.2S/1% Pt/TiO₂ in the amount of 26 mmol g⁻¹ h⁻¹ (apparent quantum efficiency was 7.7%) that exceeds recently published values for this class of photocatalysts.

The determination of the preferred location of platinum particles in TiO₂–Cd_1−xZn_xS systems was carried out for the first time.