Photocurrent Enhancement of TiO2 Nanotubes Decorated with PbS Quantum Dots

Since the discovery of photo-induced water splitting on TiO₂ electrode by Fujishima and Honda (1972) [1], TiO₂ has been considered as the most significant functional materials used in photovoltaics, photochemical water splitting, photosynthesis under sunlight spectrum [2]. However, the band gap of TiO₂ (3.2eV in phase anatase and 3.0eV in rutile) indicate primarily absorb UV radiation, which limit the efficiency of photo energy conversion in the sunlight spectrum [3]. In order to improve the utilization efficiency of sunlight, some modification technology or band structure engineering approaches on TiO₂ nanomaterials have been researched for past decades, such as gas phase doping, metal and nonmetal doping, dyes sensitization, and co-catalyst loading, etc[4]. Quantum dots (QDs) decoration is a facile technique to extent light absorption of TiO₂ to the visible region. PbS QDs is an efficient light absorbing material, which can extent light harvesting up to infrared region [5]. Therefore, the decoration of PbS QDs on TiO₂nanotubes (TNTs) is assumed to largely enhance the solar energy absorption and increase the photocurrent enhancement of TNTs photoelectrodes.

In this work, several crystallized TNTs photoelectrodes were prepared on one batch by anodization technique [6]. Three TNTs photoelectrodes decorated with different concentrated PbS QDs shown in Fig. 1a (labelled as PbS-1, PbS-2 and PbS-3) were prepared by direct absorption method, only immersing TNTs photoelectrodes into different concentrated PbS QDs solutions, which were prepared by using 5mL absolute ethanol with PbS core-type quantum dots (dissolved in toluene and purchased from Sigma-Aldrich Co.) of 50µL, 100µL and 150µL, respectively, shown in Fig. 1b. X-ray diffraction (XRD) patterns were used to determine the crystal structure of TNTs, as shown in Fig. 2. The Ti foil (Fig. 2(a)) and crystalline TNTs (Fig. 2c) after annealed 500°C for 3 hours show the strong diffraction patterns compared with the standard diffraction paterns of Ti and TiO₂ anatase. The as-prepared NTs is amorphousand the diffraction peaks are indexed to metallic substrate Ti (Fig. 2b). In addition, the diffraction patterns of crystalline TNTs decorated with PbS QDs (Fig. 2d) is similar with crystalline TNTs due to very low amounts of PbS QDs on crystalline TNTs. The photo-current measurement schematic of TNTs electrodes is shown in Fig. 3. The black light barrier taped on the ITO glass is defined the illumination area of TNTs electrodes. The I-V curves of TNTs electrodes were measured by Electrochemistry workstation (Zahner IM6) and the measured currents of  different samples are obviously raise as the concentration of PbS QDs increases in both positive and negative region. However, I-V curves of TNTs electrodes in positive region are increase faster than that in negative region. Furthermore, time dependent photorespones of different TNTs electrodes were tested under UV light with the intensity of 880.2mw/cm², as shown in Fig. 4. The higher positive bias voltage applied on TNTs photoelectrodes, the shorter photorespones time were measured.

In conclusion, we have successfully used PbS QDs as light absorber to enhance the photocurrents of TNTs photoelectrodes by direct absorption method. Improtantly, this method offers a low cost and facile process. The decoration approach and TNTs electrodes have a widely application in photovoltaic areas in the future.

Acknowledgments: The author Kang Du would like to acknowledge financial support from KD program at Buskerud and Vestfold University College, and Norwegian Research Council-FRINATEK programme (231416/F20).

References

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