Photoelectron characteristics of ZnSe quantum dots-sensitized mesoporous La-doped nano-TiO2 film

In the paper, the core-shell ZnSe quantum dots (QDs)-sensitized mesoporous La-doped nano-TiO2 thin film is prepared by a direct adsorption method. Photoelectron characteristics, photogenerated carriers transport mechanism, and microstructure of the QDs-sensitized nano-TiO2 thin film are probed via the stationary surface photovoltaic (SPV) and the transient photovoltaic technologies, supplemented by the Brunauer-Emmet-Teller adsorption isotherm technique, scanning electron microscope, Fourier transform infrared (FT-IR) absorption spectrum, and ultraviolet-visible (UV-VIS) absorption spectrum. The experimental results confirm that the surface of the nano-TiO2 film is covered with the ZnSe QDs with smaller particles by a chemical absorbing way, resulting in denser composite film of the QDs and the mesoporous nano-TiO2 than the nano-TiO2 film. In our experiment, the adsorption quantity of ZnSe QDs on nano-TiO2 film can be controlled effectively. The results show that ligand L-Cys capped at the outer layer of ZnSe QDs plays an important role in the sensitization process. Specifically, the peak of SH in the ligand disappears at 2552 cm-1 in the FT-IR spectrum of the ZnSe QDs capped by the ligand as a stabilizer. This indicates that the SH bond is broken. In the meantime, the peak of the CS stretching vibration in the ligand shifts from 638 cm-1 to 663 cm-1 due to the formation of ZnS bond. These imply that the core-shell ZnSe/ZnS/L-Cys QDs are obtained. On the other hand, according to the peak of COOH stretching vibration disappearing at 1600 cm-1 in the FT-IR spectrum of the core-shell QDs-sensitized mesoporous nano-TiO2 film, the unsaturated Ti atoms on the surface of the TiO2 film are bonded to carboxy groups from the ligand capped at the QDs. That is, the ligand acts as a bridge between the QDs and the nano-TiO2 film for achieving the sensitization. Some excellent photovoltaic characteristics of the composite film are found as follows. 1) The SPV responses of the QDs-sensitized film appear in a wavelength region of 300 nm to 800 nm (UV-VIS-Near-IR), causing the region of SPV response to enlarge about 200 nm over that of the ZnSe QDs, and 400 nm over that of the nano-TiO2 thin film. 2) The QDs-sensitized film displays an n-type photovoltaic characteristic that is different from that of the QDs. This may be more favorable for transferring those carriers from the film surface to the photo-anode material. 3) Both the separation rate and the diffusion length of photogenerated electron-hole pairs are obviously increased, and the lifetime of free charge carriers in the ZnSe QDs-sensitized film prolongs about an order of magnitude over that of the nano-TiO2 film and ZnSe QDs.