Enhanced charge-carrier transfer by CdS and Ag2S quantum dots co-sensitization for TiO2 nanotube arrays

J Colloid Interface Sci. 2015 Nov 1:457:1-8. doi: 10.1016/j.jcis.2015.06.038. Epub 2015 Jun 24.

Abstract

Thioglycollic acid was employed as a molecular linker to prepare CdS and/or Ag2S quantum dots (QDs) for the co-sensitization of TiO2 nanotube arrays through the successive ionic layer adsorption and reaction (SILAR) method. The microstructure, chemical composition, and photoabsorption of the prepared samples were analyzed by using field emission scanning electron microscopy (FE-SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-Vis diffuse reflectance spectroscopy (UV-Vis DAS). The interfacial separation and transport of photoinduced charge carries were also examined by applying current-voltage characteristics (J-V), electrochemical impedance spectroscopy (EIS), transient open circuit potential plots, and Mott-Schottky techniques. The sizes of the CdS and Ag2S QDs were found to be 6 and 4 nm, respectively. The co-sensitized samples demonstrate significantly enhanced photo absorption, greatly reduced charge transfer resistance at the semiconductor interfaces, positive shift of the flat band, elongated electron lifetimes, and accelerated interfacial separation and transport of the photoinduced charge carriers. A critical operation sequence is to first carry out the deposition of the CdS QDs that are less mismatched with TiO2 crystal lattice, followed by the deposition of Ag2S QDs. The samples prepared in this manner presented the best optoelectronic characteristics with a short photocurrent density of 9.5 mA·cm(-2). This value is 7.6% higher than 8.83 mA·cm(-2) of the sample prepared by depositing the more mismatched Ag2S QDs followed by deposition of CdS. This value is 18.8% higher than 8 mA·cm(-2) of the Ag2S-only sensitized sample, and 22.6% higher than 7.75 mA·cm(-2) of the CdS-only sensitized sample.

Keywords: Mott–Schottky; QDs co-sensitization; SILAR; Transient photovoltage.