David Hazafy

Atmospheric pressure chemical vapour deposition of boron doped titanium dioxide for photocatalytic water reduction and oxidation

Boron-doped titanium dioxide (B-TiO2) films were deposited by atmospheric pressure chemical vapour deposition of titanium(iv) chloride, ethyl acetate and tri-isopropyl borate on steel and fluorine-doped-tin oxide substrates at 500, 550 and 600 °C, respectively. The films were characterised using powder X-ray diffraction (PXRD), which showed anatase phase TiO2 at lower deposition temperatures (500 and 550 °C) and rutile at higher deposition temperatures (600 °C). X-ray photoelectron spectroscopy (XPS) showed a dopant level of 0.9 at% B in an O-substitutional position. The ability of the films to reduce water was tested in a sacrificial system using 365 nm UV light with an irradiance of 2 mW cm(-2). Hydrogen production rates of B-TiO2 at 24 μL cm(-2) h(-1) far exceeded undoped TiO2 at 2.6 μL cm(-2) h(-1). The B-TiO2 samples were also shown to be active for water oxidation in a sacrificial solution. Photocurrent density tests also revealed that B-doped samples performed better, with an earlier onset of photocurrent.

Periodate – an alternative oxidant for testing potential water oxidation catalysts

The redox catalyst ruthenium dioxide, prepared via the Adams technique, i.e. Ru(Adams), is used as a water oxidation catalyst using the oxidants (i) Ce(IV) in 0.5 M H2SO4 and (ii) periodate in 0.5 M H2SO4, water and 0.1 M KOH. Like Ce(IV), periodate is a very strong oxidant that is able to oxidise water to oxygen and can be readily monitored spectrophotometrically at 280 nm, compared with 430 nm for Ce(IV). More importantly, unlike Ce(IV), which is unstable towards hydrolysis above pH 1, periodate is stable in acid, water and strong alkali. A spectrophotometric study of the kinetics of periodate reduction, and concomitant oxidation of water to O2, reveals that in the presence of a suitable redox catalyst, Ru(Adams) in this work, periodate is able to effect the stoichiometric oxidation of water, with a turnover number > 48. In just water, the kinetics of the latter reaction appear diffusion-controlled, due to the large thermodynamic driving force, a measure of which is the difference in redox potential, i.e. ΔE = 423 mV. As this difference is decreased, ΔE = 396 mV in acid and 290 mV in strong alkali (0.1 M KOH), the kinetics become increasingly activation-controlled and slower. These findings are discussed briefly with regard to the possible use of (i) periodate as an alternative oxidant in the rapid screening of new potential water oxidation catalyst material powders that are stable only under near neutral and/or alkaline conditions, and (ii) Ru(Adams) as a benchmark catalyst.