(100f) Neutron Reflectometry Investigation of Hydrogen in Plasma Treated Hydrogen Doped Nanoporous TiO2 Thin Films for Water Splitting Photocatalysis
Although hydrogen has long been accepted as the species responsible for photocatalytic activity of hydrogenated titania (also referred to as â??black titaniaâ? because of its strong light absorbing properties), direct observation of this species has been elusive. In this work, we use neutron reflectometry (NR) to detect the hydrogen in plasma treated hydrogen doped nanoporous titania thin films. The cubic ordered nanoporous TiO2 thin films were prepared by a surfactant-templated sol-gel method and were treated with hydrogen plasma, an approach hypothesized to capitalize on the high degree of disorder in the material and the high energy of the plasma species to achieve efficient hydrogen doping. UV-vis absorbance spectra indicated that the incorporated hydrogen atoms made TiO2 films black with the enhancement of visible light absorption, and XPS analysis showed peak for Ti3+ state. The presence of hydrogen in hydrogen doped nanoporous titania (H-TiO2) films is suggested by the scattering length density (SLD) profiles obtained from NR. The SLD of H-TiO2 is much lower than that of TiO2 film due to the negative SLD of hydrogen. Thus, for the first time, NR showed that the dramatic change in visible light absorbance of H-treated black TiO2 is accompanied by significant hydrogen uptake and not just Ti3+ generation. The photocatalytic activity of hydrogen doped TiO2 (H-TiO2) films was evaluated by chronoamperometry. The effect of light sources on photoelectrochemical performance was explored using UV (365 nm) and blue (455 nm) LEDs. In photoelectrochemical water splitting, the H-TiO2 showed about 27 times and 8 times higher photocurrent under UV and blue LEDs, respectively, compared to undoped TiO2 films. Electrochemical impedance spectroscopy showed that better charge transfer is obtained in plasma treated hydrogen doped titania compared to undoped titania films. This study suggests that the dramatic change in visible light absorbance of hydrogenated black TiO2 is due to significant hydrogen uptake, and plasma-induced doping is a promising strategy to enable the efficient incorporation of heteroatoms into surfactant-templated TiO2 films, thereby leading to significant enhancement in photoelectrochemical hydrogen production.