Effects of Hydroxylation on PbS Quantum Dot Sensitized TiO2 Nanotube Array Photoelectrodes

The contact state at the heterojunction interfaces greatly influences the interfacial kinetics of the photoinduced charge carriers. In this study, we used a facile NaOH pretreatment to replenish the hydroxyl groups lost during the heat treatment for crystallization of TiO2 nanotube arrays (TNAs) prepared via anodic oxidization. By reacting the carboxylic acid groups of thioglycolic acid (TGA) with the TiO2 surface hydroxyl groups, TGA molecules were covalently linked to the TiO2 surface and then PbS quantum dots (QDs) were anchored onto the TNAs via the successive ionic layer adsorption and reaction (SILAR) method. The sample microstructure and photoelectrochemical properties were analyzed with Xray diffraction (XRD), transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), current-voltage characteristics (J-V), electrochemical impedance spectroscopy (EIS), transient photovoltage plots and Mott-Schottky curves. The contact state and electrostatic potential distribution between TiO2 {101} and PbS {111} planes were estimated by using first principle simulation. It was found that the NaOH pretreatment could enhance the crystallization degree of PbS QDs, decrease the crystal face mismatch, dangling bond density and the interfacial resistance between PbS QDs and TiO2, and accelerate the interfacial separation and transfer of photoinduced charge carriers. The first principle calculations demonstrated that the PbS QDs and TiO2 interfacial contact was strengthened, and the built-in electric field was induced from TiO2 {10 1} towards PbS {111}. These combined effects apparently improved the device photoelectrochemical performance. Compared to the sample without pretreatment, the specimen pretreated with NaOH demonstrated 19.96% and 29.93% increases in peak photoconversion efficiency after five and ten cycles of SILAR deposition, respectively. (C) 2015 Elsevier Ltd. All rights reserved.