Study On Fabrication And Photocatalytic Properties Of TiO₂Nanotube Arrays

Due to their unique properties, such as large specific surface area, highadsorption ability and excellent photoelectrochemical activities, TiO₂nanotubearrays （TNAs） have attracted significant interest for applications in many fieldsincluding solar cells, hydrogen generation, sensors, photocatalysis, pollutantdegradation and bio-applications. So far, the electrochemical anodization method isone of the most commonly used approaches to obtain TNAs. However, it is still achallenge to prepare TNAs with controllable morphologies using electrochemicalanodization method. In the present work, we focused on the synthesis of TNAs byusing electrochemical anodization method under different electrolyte systems. Theeffect of the processing parameters on the TNAs growth was systematically studiedand the formation mechanism of the TNAs was tentatively discussed. For the firsttime, TNAs with controllable morphologies were obtained using a mixed organicelectrolyte without water. In addition, as an initial exploration of theunique-structured TNAs practical application, the as-obtained TNAs were used asphotocatalysts for photocatalytic degradation of methylene blue （MB） andphotocatalytic reduction of CO₂to evaluate the photocatalytic oxidation andreduction properties, respectively. The corresponding photocatalytic mechanismswere also discussed. The main conclusions of this work are described as follows,1. The well-ordered TNAs were prepared by electrochemical anodization oftitanium foil in the SO42/F aqueous electrolyte. It was found that the growth ofTNAs was strongly influenced by the reaction conditions such as magneticstirring, reaction temperature and anodization time. The results showed thatmagnetic stirring had a negative impact on the growth of TNAs, and thetemperature of30℃was more conducive to their growth compared to15℃.In this electrolyte system, the optimal anodization time was2h under20V. Theas-prepared TNAs were identified as amorphous and could be transformed to crystalline anatase phase with no obvious change in morphology by the thermaltreatment at450℃. In addition, the formation mechanism of TNAs in theaqueous electrolyte was tentatively proposed.2. A mixed organic electrolyte of ethylene glycol/glycerol was selected to achievethe controllable preparation of TNAs for the first time. It was found that thefavorable nanotube arrays could be achieved only when the volume ratio ofethylene glycol/glycerol was1/3. In this electrolyte system, by adjusting theanodization time and voltage, the length and diameter of TNAs could be tailored,respectively. Similarly, the amorphous as-prepared TNAs could be transformedto crystalline anatase phase during the annealing process and the morphology ofTNAs kept unchanged. The formation mechanism of TNAs in the mixed organicelectrolyte was also tentatively proposed compared with that of TNAs preparedin the aqueous electrolyte.3. The photocatalytic activities of TNAs were evaluated by photocatalyticdegradation of MB and photocatalytic reduction of CO₂. The experimentalresults showed that the TNAs possessed high efficient photocatalytic properties.It was found that when compared to the commercial TiO₂P25nanoparticles,TNAs showed higher photodegradation activity during the photocatalyticdegradation of MB. Meanwhile, when using TNAs as the catalyst, CO₂could becatalytically reduced to methanol and ethanol. The UV-vis diffuse reflectancespectra revealed that compared to the commercial TiO₂P25nanoparticles, theband gap of the annealed TNAs decreased. Moreover, the catalytic mechanismof TNAs was also tentatively proposed.