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A mass of oxygen vacancies are successfully introduced into TiO2 nanotube arrays using low-cost NaBH4 as a reductant in a liquid-phase environment. By controlling and adjusting the reduction time over the range of 0-24 h, the doping concentration of the oxygen vacancy is controllable and eventually reaches saturation. Meanwhile, the thermal stability of oxygen vacancies is also investigated, indicating that part of the oxygen vacancies remain stable up to 250 °C. In addition, this liquid-phase reduction strategy significantly lowers the requirements of instruments and cost. More interesting, reduced TiO2 nanotube arrays show drastically enhanced field emission performances including substantially decreased turn-on field from 25.01 to 2.65 V/μm, a high current density of 3.5 mA/cm(2) at 7.2 V/μm, and an excellent field emission stability and repeatability. These results are attributed to the oxygen vacancies obtained by reducing in NaBH4 solution, resulting in a reduced effective work function and an increased conductivity.

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A facile rapid dehydration (RD) strategy is explored for quasi-topotactic transformation of FeOOH nanorods to robust FeO porous nanopillars, avoiding collapse, shrink, and coalescence, and compared with a conventional treatment route. Additionally, the so-called RD process is capable of generating a beneficial porous structure for photoelectrochemical water oxidation. The obtained RD-FeO photoanode exhibits a photocurrent density as high as 2.0 mA cm at 1.23 V versus reversible hydrogen electrode (RHE) and a saturated photocurrent density of 3.5 mA cm at 1.71 V versus RHE without any cocatalysts, which is about 270% improved photocurrent density over FeO with the conventional temperature-rising route (0.75 mA cm at 1.23 V vs RHE and 1.48 mA cm at 1.71 V vs RHE, respectively). The enhanced photocurrent on RD-FeO is attributed to a synergistic effect of the following factors: (i) preservation of single crystalline nanopillars decreases the charge-carrier recombination; (ii) formation of long nanopillars enhances light harvesting; and (iii) the porous structure shortens the hole transport distance from the bulk material to the electrode-electrolyte interface.