Highly Enhanced Photoactivity of Anatase TiO₂Nanocrystals by Controlled Hydrogenation-Induced Surface Defects

Abstract

Anatase TiO 2 nanocrystals (aTiO 2 ) of a uniform size have been synthesized and were subject to a successive hydrogenation under a H 2 gas flow at elevated temperatures (500−700 °C). We found that the concentration of Ti 3+ defects, such as Ti 3+ interstitials and oxygen vacancies, and their distribution between surface and bulk varied significantly, depending on the hydrogenation temperature and time. Such changes in defects were found to be critical in enhancing the photoactivity of the hydrogenated TiO 2 (H-aTiO 2 ) by an order of magnitude. In our case, H-aTiO 2 nanocrystals hydrogenated at 600 °C for longer than 10 h showed 10 times higher photoactivity than aTiO 2 , which was explained from a high surface-to-bulk defect ratio and a nonuniform distribution of defects between bulk and surface due to a preferential diffusion of bulk defects to the surface. Our study showed that a kinetically controlled hydrogenation condition could be used not only to control the surface/bulk defects but also to enhance the photoactivity of oxide nanocrystals.

Hydrogenated TiO2/SrTiO3 porous microspheres with tunable band structure for solar-light photocatalytic H2 and O2 evolution

“…5a shows the UV-Vis absorption spectra of the different samples. TiO2 and SrTiO3 have an absorption edge around 400 nm, which is consistent with traditional rutile TiO2 [27,25] and perovskite SrTiO3 [26] semiconductors, suggesting that the light irradiation for photocatalysis should derive mainly from UV light. The TiO2/SrTiO3 composite exhibits absorption up to ca.…”

“…9. As described in previous reports [24,26], hydrogenation leads to the formation of an isolated band within the band gap, which causes the band edge to red-shift toward the longer wavelength region, indicating a narrowing of the band gap [25,45]. Under visible-light irradiation, the photogenerated electrons in the covalence band of H-TiO2 and H-SrTiO3 are both excited and transferred to the corresponding conduction band (CB) (process 4).…”

“…However, the sharp strong EPR signal at g = 1.96-1.99 for the as-synthesized hydrogenated samples is assigned to a paramagnetic Ti 3+ center. Another small broad weak peak at around g = 2.00-2.02 is generally attributed to O − species, which may be generated from the interaction of O2 with the surface Ti 3+ species associated with surface oxygen vacancies [25]. Therefore, the hydrogenation of TiO2/SrTiO3 generates surface oxygen vacancies and Ti 3+ species.…”

“…Annealing the semiconductors in a H2 atmosphere creates oxygen vacancy sites, thus forming donor states below the conduction band, leading to an improvement in light absorption and water oxidation performance. Furthermore, the disordered layer produced can significantly enhance visible-light absorption [25]. The exploitation of oxygen vacancies in hydrogenated semiconductors provides new strategies to control their charge separation process and electronic structures [26,27].…”

Preparation of Blue TiO2 for Visible-Light-Driven Photocatalysis

“…In addition, hydrogenation processes require harsh synthetic conditions and/or a dangerous production process [4,10,19,[21][22][23][24][25]. Therefore, H 2 is introduced using different reducing agents such as NaBH 4 and TiH 2 [4,26,27] instead of an external dose of hydrogen gas.…”