Effect of Quantum Dot Deposition on the Interfacial Flatband Potential, Depletion Layer in TiO₂ Nanotube Electrodes, and Resulting H₂ Generation Rates

Abstract

CdS QD-modified TiO2 nanotube arrays were fabricated using an anodic oxidation-sequential chemical bath deposition process. By means of FESEM, EDS, XRD and XPS, it could be confirmed that the use of an ultrasonication-assisted deposition approach improved the distribution of CdS QDs on the tube walls. The as-prepared CdS-modified TiO2 nanotube arrays exhibited enhanced photoelectrochemical properties and hydrogen production activity, which benefitted from the extended light absorption and the improved interfacial charge-transfer properties of TiO2 nanotube arrays. By analyzing the interfacial properties, the flatband potential, the depletion layer, the capacitance, and the impedance of the CdS/TiO2 photoelectrode, it can be concluded that compared with pure TiO2 nanotube arrays the CdS QD-modified arrays exhibited a more negative flat band potential and a lower energy barrier for interfacial electron transfer. The calculated depletion layer width (dSC) of the sensitized TiO2 nanotube arrays was larger, which under a sufficiently high applied potential could facilitate the formation of a completed depletion layer; the space-charge capacitance (depletion layer capacitance) and the double-layer capacitance increased with the CdS QDs modification, and the impedance and charge-transfer resistance were reduced.