Interfacial Analysis of Anatase TiO in KOH Solution by Molecular Dynamics Simulations and Photoelectrochemical Experiments.

Hydrogen can be produced by photoelectrochemical (PEC) water splitting using a KOH solution as an electrolyte and TiO as a photoanode. In this work, we fabricated anatase TiO nanoring/nanotube arrays and TiO nanotube arrays using an anodic oxidation method, confirmed by field-emission scanning electron microscopy (FESEM) and X-ray diffractometry (XRD), and then conducted the photoelectrochemical experiments with 1 M KOH and NaSO solutions. The bias voltage, small impedance, negative flat-band potential, large capacitance, and depletion layer width in the anatase TiO-KOH system were observed, leading to the stable and large photocurrent density. To understand the effects of KOH on the interface properties of TiO/HO, the electric double layers of anatase TiO(001), (100), (101)/KOH interfaces were further investigated by calculating the ion-surface interaction with molecular dynamics simulations. It is noted that the number density of potassium ions has the same trend as that of oxygen atoms due to the layering effect in liquids and the strongest ionic hydration of K on anatase (101) is observed by analyzing the radial distribution function and coordination number. In addition, the electrostatic characteristics along the TiO/KOH interfaces were analyzed based on the Grahame double layer model. The potential drops in the outer Helmholtz layer of anatase (001), (100), and (101) surfaces are 1.08, 0.26, and 0.51 V, respectively. Compared with TiO-HO systems, the larger potential drops in the TiO-KOH system explain the phenomenon that KOH solute contributes substantially to a chemical bias in PEC reactions. At the same time, we estimated the depletion layer widths of anatase TiO(001), (100), and (101) surfaces as 37.48, 173.25, and 64.49 Å, respectively, which are of similar magnitude to the experimental results. Anatase TiO(100) with the widest depletion layer is suggested in photocatalysis. These works provide a clear understanding of interfacial behavior of KOH on anatase TiO from a microscale, which can be used to explain the promotion effect of the KOH electrolyte and guide the design of TiO nanocrystals in the PEC system.