Marginally Hydrogenated Triphasic Titania Nanotubes for Effective Visible‐Light Photocatalytic Hydrogen Generation

Abstract

We report the first demonstration of hydrogen‐annealed triphase TiO2 nanotubes (anatase–rutile–brookite) (T‐ARB) as a visible‐light‐active photocatalyst for water splitting with high quantum efficiency. As‐synthesized T‐ARB was annealed under hydrogen atmosphere at 250 and 450 °C for 2 h. We found that the tubular structures were retained in the hydrogenated TiO2 samples. It was observed that the concentrations of Ti3+ and the oxygen vacancies and their distribution significantly increased with respect to the hydrogen annealing temperature. Such changes in defects were found to be critical in enhancing the photocatalytic activity of the hydrogen‐annealed triphase TiO2 nanotubes. The triphase TiO2 nanotubes hydrogen annealed at 450 °C were 16 times more efficient than pristine TiO2. Such an increase in the photocatalytic activity was explained by a triphase band diagram with defect states below the conduction band of each phase, which facilitated visible‐light activity and easy transfer of charge carriers from one phase to another. Our study showed that the well‐designed multiphase construction with oxygen vacancies enhanced the photocatalytic activity tremendously owing to the presence of a higher number of phase junctions.

Aqueous Methanol to Formaldehyde and Hydrogen on Pd/TiO2 by Photocatalysis in Direct Sunlight: Structure Dependent Activity of Nano-Pd and Atomic Pt-Coated Counterparts

“…Moreover, an important particle size factor is considered to evaluate the hydrogen production activity of the proposed hybrid materials: the size of different morphologies (NC and TO) are incidentally the same (∼8 nm); indeed it is well-known that catalytic activity depends on particle size, which could complicate the analysis. For instance, a Pt nanocube with a 4 nm size has a 50% higher rate of solar hydrogen production compared to one with an 8 nm size. − In order to evaluate the particle size dependence for the case of Pd, we prepared Pd polycrystalline (Pd PC ) nanoparticles of ∼3–4 nm size of spherical shape, which is known to possess all the facets including (110), (111), and (100); a relevant discussion is presented below.…”

Porosity and functionality: A study on interdependence

“…Generation of TiO 2 with varied shapes and morphologies is another important aspect of synthesis. Nanorods, 35 nanowires, 36 nanotubes, 37–39 hollow nanospheres 40 and porous nanospheres 41 are few common variations. Some of these morphologies greatly boost surface area promoting better photocatalytic activities 42,43 .…”

Flash‐photoreduction method to enhance hydrogen photogeneration on Pd@TiO2

“…The relatively wide band‐gap (Eg = 3.2 eV) of bare TiO 2 limits its excitability in the UV region . It is essential to decrease the band‐gap down to below 3.0 eV in order to enhance photoexcitation under visible light . In addition, high rate of charge carriers' recombination results in a low photocatalytic efficiency of titania.…”