TiO₂ Nanotubes for Solar Water Splitting: Vacuum Annealing and Zr Doping Enhance Water Oxidation Kinetics

Abstract

Herein, we report the cooperative effect of Zr doping and vacuum annealing on the carrier dynamics and interfacial kinetics of anodized TiO2 nanotubes for light-driven water oxidation. After evaluation of different Zr loads and different annealing conditions, it was found that both Zr doping and vacuum annealing lead to a significantly enhanced light harvesting efficiency and photoelectrochemical performance. The substitution of Zr4+ by Ti4+ species leads to a higher density of surface defects such as oxygen vacancies, facilitating electron trapping on Zr4+, which reduced the charge recombination and hence boosted the charge transfer kinetics. More importantly, vacuum annealing promoted the presence of surface defects. Furthermore, the mechanistic study through impedance spectroscopy revealed that both charge transfer and surface conductivity are significantly enhanced due the presence of an oxygen-deficient TiO2 surface. These results represent an important step forward in the optimization of nanostructured TiO2-based photoelectrodes, with high potential in photocatalytic applications, including solar fuel production.

Spectacular Oxygen Evolution Reaction Enhancement through Laser Processing of the Nickel‐Decorated Titania Nanotubes

“…[ 3 ] Moreover, there are plentiful forms in which titania can be synthesized, which makes it a perfect target for further modifications. Apart from decoration with foreign species, doping, and formation of non‐stoichiometric titania, [ 4–9 ] many works rely upon the manipulation of titania architecture to improve its light‐harvesting capabilities. [ 10,11 ] The ordered titania nanotubes (TiO 2 NTs) are of particular interest, due to the straight charge percolation path, high surface area, and adjustable geometrical features.…”

Exploiting the synergistic catalytic effects of CoPi nanostructures on Zr‐doped highly ordered TiO 2 nanotubes for efficient solar water oxidation

“…TiO 2 NTs arrays were produced by electrochemical anodizing Ti foil at room temperature in two steps. 45 TNTs were ultrasonically cleaned and potentiostatically anodized for 30 minutes in an electrochemical system with a Pt foil counter electrode. The anodization process functioned at a continuous voltage of 60 V using NH 4 F, water, and EG as the electrolyte solution.…”

The role of oxygen vacancies in water splitting photoanodes

“…117,118 OVs are easily induced by the reduction of TiO 2 (TiO 2Àx ). Many synthetic routes have been reported to reduce TiO 2 , such as metal reduction, 119 ionothermal processes, 120 NaBH 4 reduction, 121 vacuum annealing 122,123 and hydrogenation. 124 The synthesis of OV-rich TiO 2 resulted in a black material with enhanced solar light harvesting and photocatalytic performance.…”

“…In a recent study, vacuum annealing and Zr doping were explored to improve the performance of nanostructured TiO 2 photoanodes. 123 Both doping and vacuum treatment led to a slightly stressed surface and consequently to a catalytically more active TiO 2 surface, concomitant with the creation of oxygen vacancies, which enhanced carrier transport. Vacuum annealing induced an anodic band shi, thermodynamically benecial for the accumulation of photogenerated holes needed for water oxidation.…”