Abstract
There is an increasing level of interest in the use of black TiO2 prepared by thermal hydrogen treatments (H:TiO2) due to the potential to enhance both the photocatalytic and the light-harvesting properties of TiO2. Here, we examine oxygen-deficient H:TiO2 nanotube arrays that have previously achieved very high solar-to-hydrogen (STH) efficiencies due to incident photon-to-current efficiency (IPCE) values of >90% for photoelectrochemical water splitting at only 0.4 V vs RHE under UV illumination. Our transient absorption (TA) mechanistic study provides strong evidence that the improved electrical properties of oxygen-deficient TiO2 enables remarkably efficient spatial separation of electron–hole pairs on the submicrosecond time scale at moderate applied bias, and this coupled to effective suppression of microsecond to seconds charge carrier recombination is the primary factor behind the dramatically improved photoelectrochemical activity.
Highlights
Titanium dioxide has been studied as a photoanode material for water oxidation for over 40 years;[1,2] the large band gap energy of TiO2 (3.0 eV, rutile) limits the maximum theoretical STH efficiency to 2.2%,3 which is well below the anticipated required STH of 10% needed for commercial viability.[4]
In light of the increased interest in hydrogen-treated TiO2 for a range of applications, including photocatalysis, DSSC, and supercapacitors,[18] it is essential that an improved understanding of the fundamental mechanisms occurring is achieved
Our mechanistic study of the factors controlling the very high STH for the oxygen-deficient rutile TiO2 nanowire arrays provides strong evidence to support the hypothesis that the improved electrical properties of H:TiO2 enables efficient charge separation under an applied bias.[17]
Summary
Titanium dioxide has been studied as a photoanode material for water oxidation for over 40 years;[1,2] the large band gap energy of TiO2 (3.0 eV, rutile) limits the maximum theoretical STH efficiency to 2.2%,3 which is well below the anticipated required STH of 10% needed for commercial viability.[4]. TiO2 has been studied with TA spectroscopy for over 25 years,[34] and slight differences in charge carrier spectra are observed, depending on the electrolyte, it is commonly accepted that on anatase TiO2, trapped photoholes absorb light at λ ∼450−550 nm, trapped photoelectrons absorb at λ∼800−900, nontrapped photoelectrons have an absorption profile that increases in intensity with wavelength (>900 nm),[35−37] and trapping of holes and electrons is known to occur within 500 ps of the laser flash.[38] Electron−hole recombination has been widely studied using TA spectroscopy across the picosecond-to-millisecond time scales, with kinetics being sensitive to the effective electron density,[5,37] and recently, the required photohole lifetime for water oxidation, that is, the lifetime of hole transfer into solution during water oxidation, has been measured and found to be ∼0.03−0.4 s, depending on the electrolyte pH.[5,39] The potential of TA spectroscopy to provide insights into the role of trap states on recombination dynamics in hydrogen-treated metal oxide photoanodes has been demonstrated by a recent ultrafast study on hydrogen-treated ZnO;[33] to the best of our knowledge, no previous studies have been reported for H:TiO2 photoanodes. We describe TA experiments on H:TiO2 photoanodes in a complete PEC cell, allowing us to elucidate the factors behind the very high IPCE values and (i) identify the critical role of hydrogen treatment on electron−hole recombination dynamics; (ii) demonstrate that photoholes generated in H:TiO2 require a lifetime for water oxidation similar to air-treated TiO2, in line with expectations for an unmodified valence band edge; and (iii) examine the factors behind the low level of visible light activity on H:TiO2
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