Study of TiO2 nanotubes decorated with PbS nanoparticles elaborated by pulsed laser deposition: microstructural, optoelectronic and photoelectrochemical properties

Titanium dioxide nanotube arrays (TiO2 NTAs) have been synthesized using the electrochemical anodization procedure. Lead sulfide nanoparticles (PbS NPs) were deposited on TiO2 NTAs (PbS NPs/TiO2 NTAs) using the pulsed laser deposition (PLD) method. The prepared samples (PbS NPs/TiO2 NTAs) were characterized using scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), UV-Vis spectroscopy and photoluminescence. The size of the PbS NPs was controlled by varying the number of laser pulses (N-LP) during the PLD process. TEM observations show that the PbS NPs are in the range of 10-20 nm, consistent with the results obtained from XRD. HRTEM and diffuse reflectivity show that, at N-LP >= 2500, the growth of the PbS NPs occurs on a previously formed PbS layer. Transmission and absorption spectra show that the PbS-NPs have an indirect optical bandgap which is particle size independent. This optical bandgap corresponds to excitonic transitions, which are greatly affected by oxygen defects, off-stoichiometry and other surface state defects, particularly for smaller NPs (N-LP<2500). The absorption spectra of the TiO2 NTAs show that the PbS NPs extend the absorption range of the TiO2-NTAs from the ultraviolet to the visible region, indicating that the PbS NPs/TiO2 NTAs heterojunction facilitates the separation of the photogenerated charge carriers. Photoelectrochemical analyses show that a maximum photocurrent current density of similar to 1.05 mA/cm(2) and a photoelectrochemical conversion efficiency of 2.5% are reached for N-LP=2500 under an illumination of 100 mW/cm(2) in the UV-Vis range.