Influence of thermal treatments in nanotubular anodic coatings of TiO2

ABSTRACT TiO2 is a semiconductor widely used in heterogeneous photocatalysis processes for water decontamination. Several studies indicate that ordered nanostructures in anatase phase improve the photocatalytic properties with respect to other morphologies and crystal structures of TiO2. The aim of the present work is to evaluate the influence of different thermal treatments on the structural integrity and crystal structure of the nanotubular TiO2 coatings synthesized by the anodic oxidation technique. A solution of 0.6% w/w ammonium fluoride and 3.5% w/w water in ethylene glycol was used as the electrolyte for the oxidation. A direct current was used at a constant voltage of 45 V during 2 h. After the anodic oxidation, thermal treatments of 2 h and 4 h at 450, 550 and 650 °C with a heating ramp of 10 ºC/min were performed. Morphology and crystalline structure of the coatings were characterized by scanning electron microscopy and X-ray diffraction, respectively. TiO2 coatings of nanotubes about 85 nm inner diameter, 18 nm wall thick and 7 ?m in length (mean value) were obtained. The nanotubes maintained their structural integrity without changes in the dimensions with the applied thermal treatments. However, the morphology of the walls of the nanotubes was modified with the time and temperature of the thermal treatments, evidencing the formation of anatase crystals. Even on the coatings thermally treated at 550 °C, nanocrystalline anatase deposits on the outside wall of the nanotubes were observed. Regarding the crystal structure, the coatings were amorphous previously to the thermal treatments and were crystalline in anatase phase after thereof, indicating the anatase stabilization even up to 650 °C, temperature at which the rutile phase appeared.

摘要：二氧化钛(TiO₂)是一种广泛应用于多相光催化水体净化过程的半导体材料。已有多项研究表明，相较于其他形貌与晶体结构的二氧化钛，锐钛矿相(anatase phase)有序纳米结构可提升其光催化性能。本研究旨在评估不同热处理工艺对阳极氧化法制备的管状二氧化钛涂层的结构完整性与晶体结构的影响。实验采用质量分数0.6%氟化铵(ammonium fluoride)与3.5%水的乙二醇(ethylene glycol)溶液作为阳极氧化电解液，以45V恒压直流电源氧化2小时。阳极氧化完成后，以10℃/min的升温速率，分别在450、550、650℃下保温2小时与4小时进行热处理。分别采用扫描电子显微镜(Scanning Electron Microscopy)与X射线衍射(X-ray Diffraction)对涂层的形貌与晶体结构进行表征。实验获得了内径约85nm、壁厚18nm、平均长度约7μm的二氧化钛纳米管涂层。经不同热处理工艺后，纳米管仍保持结构完整性，尺寸未发生变化；但纳米管的管壁形貌随热处理温度与保温时长发生改变，证实了锐钛矿晶体的生成。即使在550℃热处理的涂层上，也可观察到纳米晶锐钛矿沉积于纳米管外壁。晶体结构分析结果显示，热处理前涂层呈非晶态，经热处理后转变为锐钛矿相，表明锐钛矿相可稳定存在至650℃，该温度下金红石相(rutile phase)开始出现。