TiO_2纳米管阵列光电极的电化学阻抗及动力学特性分析

采用声电化学阳极氧化法,在无机溶剂(H3PO4+NaF水溶液)和有机溶剂(NH4F+水+乙二醇)体系电解液中加20V直流电压制得TiO2纳米管(TNT)阵列,其中无机溶剂样品(记为TNT-A)的管长为650nm,有机溶剂样品(记为TNT-E)的管长为2μm.基于X射线衍射(XRD)图谱、场发射扫描电子显微镜图(FESEM)、紫外-可见漫反射光谱(UV-VisDRS)的表征和电流-时间(I-t)曲线、Mott-Schottky图和电化学阻抗谱(EIS)的分析可知,在空气中经500℃煅烧后,TNT-E的吸光性能明显好于TNT-A的吸光性能.在紫外光((365±15)nm)辐照下,测得TNT-E的平均光电流密度与TNT-A的仅差0.05mA·cm-2,这是由于管长的增长增大了电荷转移电阻,并使得传质路径增长,增大了反应所需克服的势垒,降低了电极的反应速率,两者的电荷载流子密度分别为5.31×1020与9.86×1020cm-3.

Kinetics and Electrochemical Impedance Properties of TiO2 Nanotube Array Photoelectrode

The 2μm and 650 nm TiO₂ nanotube (TNT) arrays were fabricated by sonoelectrochemical anodic oxidation in ethylene glycol (TNT-E) and in aqueous solution (TNT-A) electrolytes at 20 V direct voltage. X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) were used to characterize the crystal phase and surface morphology of the resulting oxide films. UV-Vis diffuse reflectance spectra (UV-Vis DRS), current-time (I-t) curves, Mott-Schottky plots and electrochemical impedance spectroscopy (EIS) were used to investigate their kinetics properties and their electrochemical impedance behavior. The 2μm nanotubes of TNT-E can help to harvest more light and provide more surface active sites than the 650 nm nanotubes of TNT-A. We found that TNT-E had stronger light absorption than TNT-A after calcination in air at 500 ℃, but the photocurrent density differences between TNT-E and TNT-A was only about 0.05 mA·cm² under UV illumination ((365±15) nm). Since the longer TNT-E tubes can increase the charge transport resistance and decrease the concentration of the reactants on the electrode surface, TNT-E needs to overcome a larger energy barrier and it has a low charge carrier density of 5.31×10²⁰cm^-3. TNT-A with relatively shorter tubes showed a better kinetics property and had a charge carrier density of 9.86×10²⁰ cm^-3.