Time-dependent formation of oxygen vacancies in black TiO2 nanotube arrays and the effect on photoelectrocatalytic and photoelectrochemical properties

• The formation of oxygen vacancy was affected by anodic oxidation time and annealed atmosphere.
• The optimum anodic oxidation time of black TiO2 nanotube arrays annealing in argon atmosphere has been proved.
• The black TiO2 nanotube arrays prepared for 6 h exhibited excellent photoelectrocatalytic performance.
• A possible of formation oxygen vacancies mechanism was proposed based on the preparation experiments.

Black TiO2 nanotube arrays were synthesized via a two-step anodization procedure by controlling the duration of anodization and annealing in an argon atmosphere. Oxygen vacancies were introduced in black TiO2 nanotube arrays during the anodization and annealing process due to an insufficient oxygen atomosphere, as confirmed by EPR, XRD, SEM, TEM, XPS, and Raman analysis. Black TiO2 nanotube arrays prepared in 6 h (denoted as Ar TNT-6h) exhibited excellent photoelectrocatalytic performance and stable photoelectrochemical performance due to a considerable amount of oxygen vacancies, which ensured charge separation efficiency and strong visible light absorption. The photoelectrocatalytic activities of the samples were monitored by the decomposition of Rhodamine B (RhB). Notably, the black TiO2 nanotube arrays exhibited lower charge-transfer resistance in electrochemical impendence spectroscopy (EIS). The photocurrent density of Ar TNT-6h (1.2 mA/cm2) was nearly twelve times higher than that of air-TNTs (pristine TiO2 nanotube arrays annealed in air, 0.1 mA/cm2). The formation of oxygen vacancies in black TiO2 nanotube arrays was influenced by the duration of anodization and the annealing atmosphere. A considerable amount of oxygen vacancies significantly improved the separation efficiency of photogenerated electrons and holes, thus enhancing the photoelectrocatalytic efficiency.