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Fabrication and photoelectrochemical properties of CdSe quantum dot-sensitized TiO2 nanotube arrays Xiuchun Yang*, Wei Liu, Chao Chen, Xiaolin Cui, Peng Ren School of Materials Science and Engineering, Tongji University, Shanghai 201804, China *Corresponding author. Tel: +86-21-69580446; E-mail address:
[email protected] (X. YANG)
Abstract: CdSe quantum dot (QD) sensitized TiO2 nanotube arrays (TiO2 NTAs) were successfully prepared by successive ionic layer adsorption and reaction (SILAR) technique. Their composition, structure and photoelectrochemical property were characterized by X-ray diffractometer (XRD), field emission scanning electron microscope (FESEM), UV-vis diffuse reflectance spectroscope and electrochemical workstation. The results indicate that CdSe QDs can expand the photo-response range of TiO2 NTAs from ultraviolet region to visible region up to 736 nm with increasing SILAR cycles. The photocurrent density for the CdSe QD sensitized TiO2 NTAs dramatically increases with SILAR cycles and exhibits the highest visible photocurrent density of 11.8 mA / cm2 for 6 cycles. More SILAR cycles induce a decrease of photocurrent density. The open circuit potential of CdSe QD sensitized TiO2 NTAs decrease slowly up to 0.93 eV with increasing SILAR cycles. A model is proposed to explain the influences of SILAR cycles on the visible photocurrent density of CdSe QD sensitized TiO2 NTAs. Summary In this article, CdSe QD sensitized TiO2 nanotube arrays (TNAs) on Ti substrate are proposed based on the following reasons: (1) CdSe has a narrow bandgap (1.7 eV) and a more negative conduction band than TiO2, which can expand the photo-response range of TiO2 NTAs from ultraviolet region to visible region and improve the separation of photogenerated electrons and holes in CdSe QDs; (2) The one-dimensional structure of NTAs provides a direct electron pathway of photoinjected electrons along the TiO2 nanotubes, which significantly lower the recombination of photogenerated electron-hole pairs [1, 2]; (3) The TNAs have a direct, high quality Ohmic contact with the conductive Ti substrate after annealing since it is anodically synthesized from Ti foil; (4) Ti has higher electronic conductivity and is much cheaper than the FTO substrate, which will be advantageous for the production of low-cost large area solar cells. The SILAR method was adopted to synthesize CdSe QD sensitized TNAs because the QD-TNT contact is direct and compact, hence more favorable for charge transfer besides a satisfactory coverage of QDs inside and outside wall surfaces of the TiO2 nanotubes[3,4]. A photocurrent density of 11.8 mA/cm2 was obtained, which is the largest value until to now. TiO2 NTAs were prepared according to a two-step anodization, which was similar to our previous report
[5-7]
.
The first-step anodization was performed under 60 V for 1 h in ethylene glycol solution containing 0.5 wt.% NH4F and 3 vol.% H2O. After the first step, the as-formed TiO2 layer was peeled off by intense ultrasonication in deionized water to expose the Ti substrate. The second-step anodization was carried out in the same electrolyte for 2 h. The as-prepared samples were subsequently annealed at 4500C for 3 h after ultrasonically rinsed with deionized water. The prepared TiO2 NTAs were sensitized with CdSe QDs by using SILAR technique. First, the TiO2 NTAs were dipped into 0.01 M CdCl2 solution at 90 0C for 10 min and rinsed with deionized water. Second, it was immersed into 0.01M Na2Se solution at 90 0C for 10 min and then washed with deionized water. 0.01M Na2Se
JW3A.17
POEM 2015 © OSA 2015
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POEM 2015 © OSA 2015
was synthesized by mixing the selenium powder (0.01 M) with NaBH 4 (0.08 M) in deionized water at 90 oC and stirring for 10 min. The two-step dipping procedure was termed as one SILAR cycle. UV-Vis-NIR diffuse reflectance spectra (DRS) were recorded on a UV-4100 UV-Vis-NIR spectrophotometer with an integrating sphere attachment. Photoelectrochemical performances of the as-prepared samples were measured with an electrochemical workstation (Lanlike Company) using a three-electrode system in an electrolyte of 0.1 M Na2SO3 and 0.1 M Na2S mixing solution, where the sample as a working electrode, Pt foil as a counter electrode, and saturated calomel electrode (SCE) as a reference electrode. The working electrode was illuminated by a solar simulator equipped with a 500W Xe lamp and a visible-light filter (< 420 nm). The transient photocurrent of the working electrode was recorded according to the responses to sudden switching on and off at 0.25 V bias. The photocurrent–voltage (I–V) curves were measured under an illumination of a solar simulator at one sun (AM 1.5, 100mWcm−2). Fig.1gives UV-Vis-NIR absorption spectra, transient photocurrent density, I–V curves and schematic diagram of CdSe QD sensitized TiO2 NTAs with different SILAR cycles under visible light illumination.
Fig.1 UV-Vis-NIR absorption spectra (a), transient photocurrent densities (b), I–V curves (c) and schematic diagram (d) of CdSe QD sensitized TiO2 NTAs with different SILAR cycles Fig.1(a) indicates that the bare TiO2 NTAs absorb mainly the ultraviolet light below 387 nm due to its intrinsic band gap of 3.2 eV. After the deposition of CdSe QDs, the absorption edges are shifted significantly toward the visible region up to 736 nm with SILAR cycles, near the band gap of bulk CdSe (1.70 eV) due to the increasing QD size, and the optical absorption intensity increases with SILAR cycles due to the increasing QD content. Fig.1(b) indicates that the photocurrent density remains a constant value when the light is on, but rapidly decreases to zero as long as the light is turned off. The transient photocurrent density of all TiO2 NTAs/CdSe electrodes is timely and reproducible during the repeating on-off illuminating cycles. Significant improvement of the photocurrent density is found with increasing SILAR cycles, and the QD sensitized TiO2 NTAs with 6 cycles has the highest photocurrent density of 11.8mA/cm-2. The photocurrent
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POEM 2015 © OSA 2015
density decreases when the SILAR cycles are over 6 times. Meanwhile, I–V curves of these electrodes are measured under visible light irradiation as shown in Fig.1(c), which further confirm that the photocurrent density of CdSe QD sensitized TiO2 NTAs dramatically increases with SILAR cycles up to 6 cycles, more SILAR cycles induce a decrease of photocurrent density. The open circuit potential of CdSe QD sensitized TiO2 NTAs decrease slowly up to 0.93 eV with increasing SILAR cycles. The large open circuit potential and short circuit current density will eventually benefit the corresponding power conversion efficiency. TiO2 nanotube walls are only covered by small amounts of well-dispersed CdSe QDs for one SILAR cycle as shown in Fig.1(d)-a. The coverage of the well-dispersed CdSe QDs on the nanotube walls increases with SILAR cycles as shown in Fig.1(d)-b. The photo-generated electrons from the well-dispersed and nonagglomerated CdSe QDs are easily injected into the conduction band of TiO2 nanotubes due to the close contact between CdSe QDs and TiO2 nanotubes, and more photo-excited electrons are injected into TiO2 nanotubes with increasing CdSe QD coverage, which travel along the nanotubes to reach the external circuit. When the SILAR cycles are over 6 times, CdSe QDs agglomerate and grow as shown in Fig.1(d)-c, which induces increasing CdSe-CdSe boundaries in the TiO2 electrodes. The increased CdSe-CdSe boundaries cause an increasing loss of photo-generated electrons and holes during their transport from the CdSe QDs to the TiO2 nanotubes and the electrolyte due to increased exciton recombination and increased trapping at the interfaces. Namely, the photocurrent is decided not only by the numbers of photo-generated electrons and holes, but also by the loss of photo-generated electrons and holes during their transport process. In summary, CdSe QDs were uniformly dispersed into TiO2 nanotubes via SILAR process. CdSe QD sensitized TiO2 NTAs with 6 cycles exhibited the most excellent photoelectrochemical performances due to the extended optical absorption cooperated with the effective separation of photo-generated carriers in the photoelectrode. Acknowledgments This work was financially supported by the Nanotechnology Special foundation of Shanghai (No. 11 nm0500700). References 1. A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, P. V. Kamat, J. Am. Chem. Soc. 130, 4007(2008) 2. D. R. Baker, P. V. Kamat, Adv. Funct. Mater. 19, 805(2009) 3. H. Wang, C. Luan, X. Xu, S. V. Kershaw, A. L. Rogach, J. Phys. Chem. C 116, 484(2012) 4. H. Yang, W. Fan, A. Vaneski, A. S. Susha, W. Y. Teoh, A. L. Rogach, Adv. Funct. Mater. 22, 2821(2012) 5. Q. Y. Wang, X.C. Yang, X. Wang, M. Huang, J. W. Hou, Electrochimica Acta 62, 158(2012) 6. Q. Y. Wang, X. C. Yang, L. N. Chi, M. M. Cui, Electrochimica Acta 91, 330(2013) 7. Q. Y. Wang, X.C. Yang, D. Liu, L. N. Chi, J. W. Hou, Electrochimica Acta 83, 140(2012)
