PbS Quantum Dots and Au Nanoparticle Co-Sensitized Black TiO2 Nanotubes for Photocurrent Enhancement

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Since X. Chen et al. firstly put forward the conception of black hydrogenated TiO2, which have been attracted great interest and common concern in photocatalysis area based on the fact that the black disorder-engineered TiO2 nanomaterial have strong light absorption in visible spectrum[1]. Although black TiO2 nanomaterials have been proved with a lower bandgap [2] and a better conductivity [3] than pristine TiO2, they still could not be regarded as the single material used in photovoltaics, photochemical water splitting, and photosynthesis. In order to improve the utilization efficiency of solar energy, black TiO2 nanomaterials still need to modify by some sensitized approaches, such as metal and nonmetal doping, dyes sensitization, and co-catalyst loading method, etc[4]. Quantum dots (QDs) are nano-sized semiconductors which have been proved as an efficient sensitizer for light absorption in visible band[5]. Plasmonic effect of metal nanoparticles can also be a valid strategy to enhance the absorption by light trapping [6]. Therefore, the co-sensitized approaches on TiO2 nanotubes (TNTs) are assumed to enhance the absorption in visible region by coupled with QDs and metal nanoparticles. In this work, the TNTs were fabricated by anodization in fluoride-containing electrolyte solution. After anodiztion, the as-prepared TNTs were annealed at 500˚C in air for 3 hours to achieve anatase crystalline phase. The black TNTs were obtained by electrochemical reduction in the same electrolyte solution under the constant potential of 60V for 5 minutes. Au nanoparticles were coupled on TNTs by magnetron sputtering method and PbS QDs were loaded on black TNTs by dip coating in PbS dispersed solution which was prepared by using 5mL absolute ethanol with PbS core-type quantum dots (dissolved in toluene and purchased from Sigma-Aldrich Co.) 150µL. The surface morphologies of samples were characterized by using scanning electron microscopy (SEM, FEI Quanta FEG 600) and light absorption properties were performed by UV-vis spectrophotometers (Shimadzu UV-2600). All electrochemical performance of samples were evaluated in 0.5M Na2SO4 solution by electrochemical workstation (Zahner elektrik IM6) in a three-electrode configuration, with a Pt coil as counter electrode and a Ag/AgCl (saturated KCl filling solution) as reference electrode. Fig. 1(a) and (c) show the SEM images of PbS quantum dot and Au nanoparticle co-sensitized black TiO2 nanotubes, indicating the diameter of TNTs are approximate to ~230nm and the Au nanoparticles are mainly loaded on the circule orifice of TNTs compared with the morphologies shown in Fig. 1(b). Fig. 2 shows the measured absorption spectra of photoelectrodes. The pristine TNTs show the weak absorption in visible light region above 400nm, while the absorptive spectra of sensitized photoelectrodes have sharply enhanced in the wavelength of 400~850nm. Although absorbance of the photoelectrodes loaded with Au nanoparticles are reduced from 600nm to 850nm, these samples show the strong absorption in the wavelength of 400~600nm. Fig. 3 shows the photocurrent properties of electrodes recorded by linear sweep voltammograms in dark and at UV illumination of 880mW/cm2. Under illumination, the sensitized samples exhibit substantially enhanced photocurrent at the potential from +0.6V to +1.5V compared with the pristine TNTs. In conclusion, we have successfully prepared the co-sensitized black TNTs photoelectrodes using PbS quantum dot and Au nanoparticles as light absorber for photocurrent enhancement. The preliminary testing results exhibit strong light absorption in visible region and we expect these sentisized approaches could be applied in photovoltaic areas in the future. Acknowledgments: The author Kang Du acknowledge financial support from KD program at University College of Southeast Norway, Norwegian Research Council-FRINATEK programme (231416/F20), EEA-Poland (237761) and partial funding for this work was obtained from the Norwegian PhD Network on Nanotechnology for Microsystems, which is sponsored by the Research Council of Norway, Division for Science, under contract no. 221860/F40.