Reduced TiO2nanotube arrays for photoelectrochemical water splitting

Abstract

We report a facile one-step chemical method to synthesize partially reduced TiO₂ nanotube arrays (NTAs). The NaBH₄ treatment introduces oxygen vacancies on the surface and interior of TiO_2. Oxygen vacancy extends the photocatalytic activity of TiO₂ NTAs from the UV to visible light region, and enhances the electrical conductivity as well as charge transportation. Surface oxygen vacancies serve as charge carrier traps as well as adsorption sites where the charge transfer to adsorbed species inhibits the surface charge recombination, whereas bulk oxygen vacancies tend to act as charge carrier traps where e–h recombination occurs. The optimally reduced TiO₂ NTAs yield a photocurrent density of 0.73 mA cm⁻² at 1.23 V_RHE and a highest photoconversion efficiency of 1.31% at a rather low bias of 0.40 V_RHE under a standard AM 1.5G solar illumination. Not only does the incident photon to current conversion efficiency (IPCE) spectrum increase in the UV region, but photoactivity in visible light also emerged. Surface oxygen vacancies, serving as electron donors, cause a noticeable negative flatband shift and increase the donor density of TiO₂ NTAs 2-fold. Electron paramagnetic resonance (EPR) spectra confirm the presence of oxygen vacancies on the surface and interior of TiO₂. Benefitting from the oxygen vacancy, a narrowed band gap of 2.46 eV and suitable localized states for hydrogen production are observed.