Interfacial Analysis of Anatase TiO2 in KOH Solution by Molecular Dynamics Simulations and Photoelectrochemical Experiments

摘要

Hydrogen can be produced by photoelectrochemical (PEC) water splitting using a KOH solution as an electrolyte and TiO_(2) as a photoanode. In this work, we fabricated anatase TiO_(2) nanoring/nanotube arrays and TiO_(2) 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 Na_(2)SO_(4) solutions. The bias voltage, small impedance, negative flat-band potential, large capacitance, and depletion layer width in the anatase TiO_(2)–KOH system were observed, leading to the stable and large photocurrent density. To understand the effects of KOH on the interface properties of TiO_(2)/H_(2)O, the electric double layers of anatase TiO_(2)(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_(2)/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_(2)–H_(2)O systems, the larger potential drops in the TiO_(2)–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_(2)(001), (100), and (101) surfaces as 37.48, 173.25, and 64.49 ?, respectively, which are of similar magnitude to the experimental results. Anatase TiO_(2)(100) with the widest depletion layer is suggested in photocatalysis. These works provide a clear understanding of interfacial behavior of KOH on anatase TiO_(2) from a microscale, which can be used to explain the promotion effect of the KOH electrolyte and guide the design of TiO_(2) nanocrystals in the PEC system.