Magneli phases doped with Pt for photocatalytic hydrogen evolution
Abstract
Defined substoichiometric titanium oxides (TixO2x-1 with 3 < x < 10) called Magneli phases have been investigated mostly for their unusual high conductivity and metal-like behavior. In photocatalysis, Magneli phase containing titania particles have been reported to provide favorable charge separation resulting in enhanced reaction efficiency. In the current work we describe a one-step synthesis of Magneli-containing mixed phase nanoparticles that carry directly integrated minute amounts of Pt. Phase optimized nanoparticles that contain only a few hundred ppm Pt are very effective photocatalysts for H2 evolution (they provide a 50-100 times higher H2 evolution than plain anatase loaded with a similar amount of Pt). These photocatalysts are synthesized in a setup combining a hot-wall reactor that is used for TiOx synthesis with a spark generator producing Pt nanoparticles. Different reactor temperatures result in various phase ratios between anatase and Magneli phases. The titania nanoparticles (ca. 24 - 53 nm) were characterized using XRD, HRTEM, XPS and EPR spectra as well as ICP-OES analysis. The best photocatalyst prepared at 900 (which consists of mixed phase particles of 32% anatase, 11% rutile and 57% Magneli phases loaded with 290 ppm of Pt) can provide a photocatalytic H2 evolution rate of ca. 5432 micromol h-1 g-1 for UV and ca. 1670 micromol h-1 g-1 for AM1.5 illumination. For powders converted to higher amounts of Magneli phases (1000 and 1100), a drastic loss of the photocatalytic H2 generation activity is observed. Thus, the high photocatalytic efficiency under best conditions is ascribed to an effective synergy between multi-junctions of Magneli titania and Pt that enable a much more effective charge separation and reaction than conventional Pt/anatase junctions.