Studies of Growth and Blackening Mechanisms of Anodic TiO2

陽極氧化鈦的生長和變黑機理研究

Student thesis: Doctoral Thesis

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Award date11 Aug 2020

Abstract

Titanium dioxide (TiO2) as a type of important semiconductor has been regarded as one of the most promising materials used in energy- and environment-related areas, including supercapacitors, photocatalysis, solar cells, solar water splitting, etc. Remarkably, anodic 1-D aligned TiO2 nanotube arrays (TNTAs) with excellent electronic and optical properties, such as, high light absorption and current collection efficiency, low surface recombination and long electron diffusion length, determined by special unidirectional architecture have attracted tremendous interest. However, on the one hand, the fundamental exploration of growth mechanism behind the morphological complexities is limited, and the effects of electrochemical parameters (e.g., anodizing current, voltage) on porous nanostructures lack reasonable explanations and evidences in situ; on the other hand, the wide bandgap of TiO2 is only capable of absorbing the UV light which accounts for only ~ 5% of the solar energy, greatly limiting its photocatalytic efficiencies. So, systematic study of the formation mechanism of anodic oxide and exploring the method for breaking through its intrinsic limits of photocatalytic efficiencies are necessary.

In this thesis, we mainly focused on anodic TiO2 nanotubes, firstly, the growth mechanism of TNTAs was discussed and a new growth mechanism of TNTAs was proposed. Then, a facile, low-cost solution-based method was used to fabricate black TNTAs, further, the reaction mechanism, environmental-related applications and expanding research were explored in detail.

The opening chapter briefly introduces different explanations for TNTAs growth mechanism, different methods to fabricate black TiO2 nanomaterials and their applications in different areas and different mechanism research for black TiO2 formation.

It has puzzled scientists for decades why anodic TNTAs, a most intensively studied anodic oxide, displayed a distinct nanotubular morphology, ., anodic Al2O3 (AAO), another important anodic oxide, features nanochannels. Different tube-formation mechanisms have been proposed but the enigma remains unsolved. Chapter 2 elucidates that the tube-formation in anodic TNTAs is due to evolution of the metastable titanium oxyfluorides initially produced at the anodic oxide/Ti interfaces, leading to gaps in between the walls of the neighboring nanochannels.

Then, a convenient, low-cost, and safe method for producing black TNTAs was presented. By a simple soaking in an ethylene glycol (EG) solution of NH4F, the as-anodic TNTAs turn black, with a substantial amount of oxygen vacancies and Ti3+ sites induced. The thus obtained black TNTAs deliver greatly enhanced photoelectrochemical and photocatalytic performance of high stability.

Finally, the formation mechanism for blackening TiO2 through an easy solution-soaking treatment at mild conditions (e.g., 60 °C) in polyol (e.g., ethylene glycol) solutions was disclosed. The F- ions, acidic polyol solvents, and contact between the TiO2 and Ti components are discovered to be the prerequisites for the water-promoted comproportionation between TiO2 and Ti, which generate Ti (III) species that effectively blacken different TiO2 materials--including the anodic nanotube arrays and commercial P25 powders. The black TiO2 thus obtained from this convenient soaking method displays greatly improved photoelectrochemical performance.