Structural engineering of Ti-Mn bimetallic phosphide nanotubes for efficient photoelectrochemical water splitting

Abstract

Designing next-generation advanced electrode materials by engineering their structural and compositional features can provide a feasible strategy to enhance the electrochemical performance of energy conversion devices. In this study, the rational pathway to design and fabricate nanotube arrays of titanium manganese phosphide via etching of titanium-manganese alloy followed by plasma phosphidation in PH3 environment is presented and discussed. The structural and elemental analyses of the air-annealed electrodes before plasma treatment confirmed the presence of different binary oxides; TiO2, MnO, and Mn2O3. However, the XPS fitting showed the presence of Ti3+ and higher ratio of MnO when annealed in hydrogen atmosphere. The presence of composite oxides resulted in a band gap reduction, which increased the light harvesting capability of the material. This synergetic effect resulted also in a shift in the open-circuit voltage (VOC) and almost 10-fold increase in the photocurrent density compared to the performance of the nanotubes annealed in air. Mott-Schottky analysis showed a four-orders of magnitude enhancement in the carrier density for the electrodes annealed in Hydrogen and treated in PH3-plasma compared to those annealed in O2 or air, ascribed to the creation of Ti3+ defects and phosphidation. Our study thus paves the way to a new approach for creating high-performance hybrid electrodes for PEC water splitting.