Engineering titania nanostructure to tune and improve its photocatalytic activity

Abstract

Photocatalytic pathways could prove crucial to the sustainable production of fuels and chemicals required for a carbon-neutral society. Electron−hole recombination is a critical problem that has, so far, limited the efficiency of the most promising photocatalytic materials. Here, we show the efficacy of anisotropy in improving charge separation and thereby boosting the activity of a titania (TiO 2 ) photocatalytic system. Specifically, we show that H 2 production in uniform, one-dimensional brookite titania nanorods is highly enhanced by engineering their length. By using complimentary characterization techniques to separately probe excited electrons and holes, we link the high observed reaction rates to the anisotropic structure, which favors efficient carrier utilization. Quantum yield values for hydrogen production from ethanol, glycerol, and glucose as high as 65%, 35%, and 6%, respectively, demonstrate the promise and generality of this approach for improving the photoactivity of semiconducting nanostructures for a wide range of reacting systems.titania | brookite | photocatalysis | photoreforming | hydrogen

Brookite: Nothing New under the Sun?

“…On brookite TiO2 nanorods, reduction and oxidation reactions were observed to proceed predominantly on {210} and {212} The EPR results further indicated that electrons in longer nanorods transfer to Pt faster than electrons in shorter nanorods. The capture of electrons by Pt increases the lifetime of the charge-separated state, as indicated by the much faster decays observed for nanorods without Pt in ethanol [28]. A similar effect was observed for Au surface decoration of brookite nanorods [66], which enabled electrons a lifetime of four orders of magnitude longer due to efficient hopping on brookite lateral facets.…”

“…The capture of electrons by Pt increases the lifetime of the chargeseparated state, as indicated by the much faster decays observed for nanorods without Pt in ethanol [28]. A similar effect was observed for Au surface decoration of brookite nanorods [66], which enabled electrons a lifetime of four orders of magnitude longer due to efficient hopping on brookite lateral facets.…”

“…As recently demonstrated by some of the authors by a combination of electron paramagnetic resonance (EPR) spectroscopy and in-situ TRAS [28], H 2 production rates in the photoreforming of ethanol, glucose and glycerol increases with the increasing length of Pt-decorated brookite nanorods due to the longer lifetime of charge-separated states (Figure 4). A broad photoinduced absorption feature in the visible range is obtained in TRAS when pumping above the brookite band gap, which was assigned to trapped holes lying energetically within the band gap and physically on or near the surface of the TiO 2 nanoparticles [63].…”

“…The nanostructure also plays a pivotal role in photocatalysis. For instance, the rate of electron-hole recombination in brookite nanorods can be controlled by engineering their length, as recently reported by some of us [28]. The synthesis of nanostructured TiO 2 has been widely investigated in the literature [18,20,55], so that a wide library of morphology-controlled materials can now be produced.…”

“…HRTEM can be used to study preferential exposure of facets, crystal morphology, grain boundaries and presence of defects, which can all influence the photocatalytic activity of a material and are therefore very important in structure/activity relation studies. Nonetheless, in some cases it is hard to distinguish between TiO 2 phases, making a definitive assignment difficult if nanocrystals are not suitably aligned with respect to the electron beam [28]. Moreover, the presence of defects and lattice deformation can lead to slight changes of diffractograms, so intensive characterization is needed in this kind of HRTEM study [26].…”