Synthesis and photoelectrochemical response of CdS quantum dot-sensitized TiO2 nanorod array photoelectrodes
© Hu et al.; licensee Springer. 2013
Received: 2 March 2013
Accepted: 1 May 2013
Published: 10 May 2013
A continuous and compact CdS quantum dot-sensitive layer was synthesized on TiO2 nanorods by successive ionic layer adsorption and reaction (SILAR) and subsequent thermal annealing. The thickness of the CdS quantum dot layer was tuned by SILAR cycles, which was found to be closely related to light absorption and carrier transformation. The CdS quantum dot-sensitized TiO2 nanorod array photoelectrodes were characterized by scanning electron microscopy, X-ray diffraction, ultraviolet–visible absorption spectroscopy, and photoelectrochemical property measurement. The optimum sample was fabricated by SILAR in 70 cycles and then annealed at 400°C for 1 h in air atmosphere. A TiO2/CdS core-shell structure was formed with a diameter of 35 nm, which presented an improvement in light harvesting. Finally, a saturated photocurrent of 3.6 mA/cm2 was produced under the irradiation of AM1.5G simulated sunlight at 100 mW/cm2. In particular, the saturated current density maintained a fixed value of approximately 3 mA/cm2 without decadence as time passed under the light conditions, indicating the steady photoelectronic property of the photoanode.
KeywordsQuantum dots CdS Nanocable arrays SILAR
The quantum dot-sensitized solar cell, which may be considered as the third generation of solar cells, has attracted great scientific and industrial interest in recent years [1–3]. Inorganic quantum dots (QDs), such as CdS [4–6], CdSe [7, 8], and CdTe , have the following advantages as sensitizers: an effective bandgap controlled by the size of the QDs, large absorption of light in the visible region, and the possibility for multiple exciton generation. Among the various QD materials, CdS has been receiving much attention because of its high potential in photoabsorption in the visible region. Thus, CdS has been widely studied and applied to light-emitting diodes , biology applications , and solar cells [12, 13]. CdS QDs are prepared using several methods, including thermal evaporation , spray pyrolysis , chemical bath deposition (CBD) , and successive ionic layer adsorption and reaction (SILAR) . Among these methods, SILAR is the most commonly used given its simple technique and capacity to produce high-quality nanoparticles in large scale.
One-dimensional (1D) single-crystalline oxide array is very popular because of its higher specific surface area than that of its film, its ability to grow easily over a large area on the substrate, as well as its bandgap that can match well with CdS. Several studies on 1D single-crystalline oxide array have been reported [18, 19]. Yao et al.  reported on CdS QD-sensitized ZnO nanorod arrays (NRAs) that displayed a power conversion efficiency of 1.07%. CdS QD-sensitized TiO2 NRA solar cells have been prepared through the CBD method with a photocurrent intensity of 5.13 mA/cm2 at 0-V potential and an open-circuit potential of −0.68 V . We have synthesized various sizes of CdS QDs and dye-co-sensitized TiO2 NRA solar cells by SILAR, yielding a power conversion efficiency of 2.81% . In the present study, the photoelectrochemical properties and stability of the TiO2/CdS core-shell NRA photoelectrode were studied. In our experiment, TiO2 nanorods were prepared through the hydrothermal method without a seed layer, and the CdS QDs were synthesized by SILAR. The optimum CdS QD-sensitized TiO2 NRA photoelectrode that formed the TiO2/CdS core-shell structure with a shell thickness of 35 nm was fabricated by SILAR in 70 cycles and then annealed at 400°C for 1 h in air atmosphere. This photoelectrode presented an improvement in light harvesting, ultimately producing a saturated photocurrent of 3.6 mA/cm2 under the irradiation of AM1.5G simulated sunlight at 100 mW/cm2. In particular, the saturated current density maintains a fixed value of approximately 3 mA/cm2 without decadence as time passed under the light conditions, indicating the steady photoelectronic property of the photoanode.
TiO2 NRAs were prepared through the hydrothermal method. Approximately 8 mL of deionized water was mixed with 8 mL of concentrated hydrochloric acid (36.5% to 38% by weight) to reach a total volume of 16 mL. The mixture was stirred in air for 5 min. Then, 0.2 mL of titanium butoxide was added into the solution, which was stirred for another 5 min. A fluorine-doped tin oxide (FTO) substrate (approximately 2 cm × 2 cm) was placed in a 20-mL autoclave. The hydrothermal method was used to grow the TiO2 NRAs at 150°C for 10 h. Samples were annealed at 500°C for 2 h in air. CdS QDs were deposited on the TiO2 nanorods through SILAR. The FTO substrate grown with TiO2 NRAs was immersed in a 0.3 mol/L Cd(CH3COO)2 aqueous solution for 2 min, rinsed with deionized water, then immersed for another 2 min in a 0.3 mol/L Na2S aqueous solution, and rinsed with deionized water. The above series of steps were carried out to prepare the CdS QDs, and these steps were repeated several times until a thin layer of quantum dots was formed. The samples were then annealed at 400°C for 1 h in air atmosphere.
The morphology of the sample was studied by scanning electron microscopy (FE-SEM; JEOL JSM-6700F, Akishima-shi, Japan). The structure and crystallinity of the samples were investigated by X-ray diffraction (XRD; D8, Bruker AXS, Inc., Madison, WI, USA). The optical properties of the samples were characterized by ultraviolet–visible (UV–vis)-IR absorption (UV360 spectrometer, Shimadzu, Corporation, Kyoto, Japan). The microstructure of a single nanorod was observed by transmission electron microscopy (TEM; FEI TECNAI G20, Hillsboro, OR, USA). Photoelectrochemical measurements were performed in a sulfide/polysulfide (S2−/Sn2−) electrolyte containing 0.5 M S and 0.3 M Na2S dissolved in deionized water, in which the TiO2/CdS arrays on FTO, Pt foil, and SCE were used as the working, counter, and reference electrodes, respectively. The illumination source used was AM1.5G light at 100 mW/cm2.
Results and discussion
A simple SILAR method was used to prepare a CdS shell on TiO2 NRAs. The optimum sample was fabricated by SILAR in 70 cycles and then annealed at 400°C for 1 h in air atmosphere, providing an improvement of light harvesting and ultimately yielding a saturated photocurrent of 3.6 mA/cm2 under the irradiation of AM1.5G simulated sunlight. In particular, the saturated current density maintains a fixed value of about 3 mA/cm2 without decadence as time passed under the light conditions, indicating the steady photoelectronic property of the photoanode.
This work was supported in part by the National Nature Science Foundation of China (no. 51072049), the Research Fund for the Doctoral Program of Higher Education of China (RFDP; no. 20124208110006), and the NSF and ED of Hubei Province (nos. 2009CDA035, Z20091001, and 2010BFA016).
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