(621bc) Length-Dependent Photoreactivity in Highly Active Brookite Titania Nanorods

Introduction

Titanium dioxide (TiO₂, titania) nanomaterials are the most studied photocatalysts because of their abundancy, non-toxicity, stability and activity [1]. Anatase and rutile polymorphs have been deeply investigated because they are the energetically favored forms at the nanoscale and in the bulk, respectively [2]. Brookite, instead, has been rarely studied, despite theoretical and experimental data support its higher activity in some photocatalyzed transformations [3]. Although the relatively large band-gap (E_g ~3.0-3.2 eV) does not allow its function using the visible part of the solar spectrum, its structural modification can boost the efficiency under UV illumination [4]. Here, we show that brookite titania nanorods are highly active photocatalysts for hydrogen production from photoreforming [5], providing a new paradigm for the preparation of highly active systems to harvest and store solar energy.

Materials and Methods

Brookite nanorods were prepared following the synthesis devised by Buonsanti et al. [6] with few modifications [7]. 30 mmol of oleylamine, 10.2 mL of 1-octadecene, and 0.48 mL of oleic acid are combined and degassed at 120 °C for 1 h. After degassing the flask, 1.5 mL a stock solution of TiCl₄ (0.2 M) in oleic acid (1 M) in 1-octadecene is added at 60 °C. Then, the solution is quickly heated to 290 °C and held 10 min to allow for the formation of seed crystals. Variable portions of the same TiCl₄ stock solution is then pumped into the flask kept at 290 °C at 0.3 mL min⁻¹ depending on the desired length needed. Afterward, the heating mantle is removed and the flask is left to cool naturally to ambient temperature.

Results and Discussion

The nanorods show quite uniform (10-12% dispersion) and clearly distinct lengths that vary from 25 to 45 nm. The organic ligands can be removed by appropriate treatments  and isolated, dried and used as powders. X-Ray diffraction data of such powders supports the formation of pure-phase brookite rods. The bandgap of the rods at varying lengths after ligand exchange with NOBF₄ was about 3.4 eV for all the samples, larger than rutile and anatase polymorphs, is in accordance with previous works on brookite materials.

The nanorods are also colloidally stable thanks to the presence of oleylamine and oleic acid ligands on their surface. This element allows for the preparation of smooth and uniform thin films by deposition of the rods from concentrated colloidal dispersions. Because of kinetic barriers, a metal such as Pt is usually deposited onto the titania surface to enhance photocatalytic rates. In order to overcome catalytic activity disparities because of Pt particle size differences using conventional impregnation or photodeposition methods, we co-deposited the titania rods with pre-synthesized Pt nanocrystals with uniform particle size distribution of 4.5 ± 0.4 nm. The catalytic activity increases with increasing length of the rods up to about 30 nm, to then slightly level or decrease for longer rods. To explain the catalytic activity, an increase of the available surface area as a consequence of the reduction in particle size would be against the observed catalytic trend, favoring smaller rods, therefore excluding its main influence to the observed activity. To clarify the role of excited carrier dynamics in the photocatalytic reaction, we carried out in-situ ultrafast transient absorption and in-situ EPR studies of the films under similar conditions used for the photocatalytic experiments. In the case of ethanol, photooxidation by trapped holes has been observed to occur in the ns time scale, and we therefore followed the transient absorption signal of trapped holes for the rods of different lengths. We observed a clear dependence of the bleaching feature for trapped holes on the nanorods length, with longer rods possessing longer recombination times. The hole diffusion length in titania is about 10 nm, but the electron diffusion length can be of the order of micrometers. Because of the 1-D nature of the nanorods, we therefore attribute the higher catalytic activity of the longer rods to the improvement in electron-hole separation. The maximum in catalytic activity is however observed at intermediate sizes, because a trade-off between activity and surface area is observed.

Significance

The study clearly reveals that 1-D brookite materials can be highly active for important reactions such as photocatalytic hydrogen production from renewable compounds. Additionally, it has been shown that by tailoring the nanostructure, the activity can be improved dramatically obtaining one of the most active photocatalytic systems under Solar simulated irradiation to date.

References

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