Resumo:
Abstract
Hydrogen fuels generated by water splitting using a photocatalyst and solar irradiation are
currently gaining the strength to diversify the world energy matrix in a green way. CdS quantum
dots have revealed a hydrogen generation improvement when added to TiO2 materials under
visible-light irradiation. In the present paper, we investigated the performance of TiO2 nanotubes
coupled with CdS quantum dots, by a molecular bifunctional linker, on photocatalytic hydrogen
generation. TiO2 nanotubes were obtained by anodization of Ti foil, followed by annealing to
crystallize the nanotubes into the anatase phase. Afterwards, the samples were sensitized with
CdS quantum dots via an in situ hydrothermal route using 3-mercaptopropionic acid as the
capping agent. This sensitization technique permits high loading and uniform distribution of CdS
quantum dots onto TiO2 nanotubes. The XPS depth profile showed that CdS concentration
remains almost unchanged (homogeneous), while the concentration relative to the sulfate anion
decreases by more than 80% with respect to the initial value after ∼100 nm in depth. The
presence of sulfate anions is due to the oxidation of sulfide and occurs in greater proportion in
the material surface. This protection for air oxidation inside the nanotubular matrix seemingly
protected the CdS for photocorrosion in sacrificial solution leading to good stability properties
proved by long duration, stable photocurrent measurements. The effect of the size and the
distribution of sizes of CdS quantum dots attached to TiO2 nanotubes on the photocatalytic
hydrogen generation were investigated. The experimental results showed three different
behaviors when the reaction time of CdS synthesis was increased in the sensitized samples, i.e.
similar, deactivation and activation effects on the hydrogen production with regard to TiO2
nanotubes. The deactivation effect was related to two populations of sizes of CdS, where the
population with a shorter band gap acts as a trap for the electrons photogenerated by the
population with a larger band gap. Electron transfer from CdS quantum dots to TiO2
semiconductor nanotubes was proven by the results of UPS measurements combined with
optical band gap measurements. This property facilitates an improvement of the visible-light
