Photocurrent Properties of Ti3+ Introduced TiO2 Nanotubes Photoelectrode Decorated by CdSe Quantum Dots
Authors: Du Kang , Liu Guohua , Chen Xuyuan , Wang Kaiying 
Published: February 2020

TiO2 nanotubes (TNT) is one type of pristine TiO2 nanomaterials, which is active under ultraviolet (UV) irradiation for photoelectric/photocatalytic applications [1]. The photocatalytic activity of TiO2 is strongly dependent on the surface defects: Ti3+ defects and oxygen vacancies [2]. Ti3+ defect sites are obtained by several reduction methods to form TiO2-x [3], which has been demonstrated to exhibit good performance in visible light absorption [4]. Semiconductor quantum dots (QDs) such as CdSe QDs, with tunable band gaps offer new opportunities for harvesting light energy in the visible and infrared regions of solar light [5]. Therefore, the decoration of CdSe QDs on TiO2-xTNTs is assumed to largely enhance the solar energy absorption and increase the photocatalytic activities of TNT.
In this work, TNT samples were fabricated by anodization technique in fluoride-containing electrolyte solution (97 vol% EG, 3 vol% DI water, and 0.5wt% NH4F) and Ti3+ sites were introduced into TNT (symbolized as T-TNT) by electrochemical reduction in the same electrolyte solution [6]. Three different CdSe QDs solutions, denoted as CdSe-1, CdSe-2, and CdSe-3, were prepared by dispersing 50µL CdSe core-type QDs (5mg/mL, dissolved in toluene and purchased from Sigma-Aldrich Co.), into 3mL absolute ethanol, respectively. TNT and T-TNT samples were decorated by dropping 150µL CdSe QDs solutions in 5 cycles, approximately 12.5µg CdSe QDs were loaded onto samples. Fig.1 (a) shows the scanning electron microscope (SEM) image of TNT, indicating the diameter of TNT are around ~150 nm. Different CdSe QDs were dispersed onto a thermal oxide (SiO2) silicon wafer and characterized by atomic force microscope (AFM), suggesting the size of monodisperse CdSe QDs are around 10~30 nm, shown in Fig. 1 (b), (c), and (d), respectively. Fig. 2 shows the absorption spectra of different CdSe QDs. The absorption peaks of different CdSe QDs are 425 nm, 510 nm, and 630 nm, and the corresponding bandgaps are 2.34eV, 2.15eV, and 1.76eV, respectively. The color change of CdSe QDs solutions under visible light and UV fluorescent indicate the photoexcitation properties of CdSe QDs, shown in the inset image of Fig. 2. Fig. 3 shows the absorption spectra of W-TNT and T-TNT decorated by different CdSe QDs and the inset image shows the color of T-TNT turns to black or dark blue after introducing Ti3+sites in the surface of TNT. T-TNT samples show obvious absorption enhancement in the wavelength from 400nm to 850nm. After decorated with CdSe QDs, TNT and T-TNT show similar enhancement from 400 nm to 675 nm and the bandgap of samples can narrow down to 2.86eV (T-TNT+CdSe-3) from 3.07eV (pristine TNT).
In conclusion, the aborption properties of TNT and T-TNT photoelectrodes are enhanced by introducing Ti3+ sites and decorating with different size types of CdSe QDs as light absorber. Improtantly, we expect TNT based nanocomposites could offer a low cost and high efficiency co-catalyst systems used in photovoltaic, photocatalytic water splitting, and photocatalytic CO2reduction.

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

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Figure 1