Subpicosecond to Second Time-Scale Charge Carrier Kinetics in Hematite–Titania Nanocomposite Photoanodes

Abstract

Water splitting with hematite is negatively affected by poor intrinsic charge transport properties. However, they can be modified by forming heterojunctions to improve charge separation. For this purpose, charge dynamics of TiO2:α-Fe2O3 nanocomposite photoanodes are studied using transient absorption spectroscopy to monitor the evolution of photogenerated charge carriers as a function of applied bias voltage. The bias affects the charge carrier dynamics, leading to trapped electrons in the submillisecond time scale and an accumulation of holes with a lifetime of 0.4 ± 0.1 s. By contrast, slower electron trapping and only few long-lived holes are observed in a bare hematite photoanode. The decay of the long-lived holes is 1 order of magnitude faster for the composite photoanodes than previously published for doped hematite, indicative of higher catalytic efficiency. These results illustrate the advantages of using composite materials to overcome poor charge carrier dynamics, leading to a 30-fold enhancement in photocurrent.

Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodes

“…The transient absorption decays and decay component spectra of the reference hematite are a good match to those we have published earlier for TiO 2 -doped hematite photoanodes, even though the electron trapping lifetime of our reference hematite is longer than for the TiO 2 -doped samples (6.4 ms vs. 0.3 ms, respectively). 48 We attribute this difference to the faster electron extraction dynamics from the surface states due to the preferable band alignment of the TiO 2 -doped material. 49 However, the water oxidation dynamics are practically identical (exponential lifetimes 405 ms and 400 ms).…”

“…25 Transient absorption spectroscopy (TAS) performed under operating conditions has been used to show ultrafast sub-picosecond electron trapping into a surface species on the hematite surface, with the trapping extending well into the millisecond timescale. 47,48 This trapping is observed as a bleaching (i.e. reduced absorption compared to the ground state) at 575 nm when an anodic bias voltage is applied.…”

“…We have illustrated earlier that an increase in this electron trapping results in the formation of an increased amount of long-lived holes in the hematite surface, resulting in increased photoelectrochemical performance. 48 Thus, we performed submillisecond to second timescale TAS measurements under operating conditions to elucidate on the differences in charge carrier dynamics and electron trapping between our photoanodes.…”

“…The fast bleach is attributed to prolonged electron trapping into the surface states that we are able to observe with EIS, whereas the positive transient absorption is attributed to long-lived holes in the hematite surface that partake in the water oxidation reaction. 47,48 The bleach is a localised band with a minimum at 575 nm, whereas the positive transient absorption is observed in the whole monitored region. The transient absorption decays and decay component spectra of the reference hematite are a good match to those we have published earlier for TiO 2 -doped hematite photoanodes, even though the electron trapping lifetime of our reference hematite is longer than for the TiO 2 -doped samples (6.4 ms vs. 0.3 ms, respectively).…”

“…This is indicative of reduced recombination of the long-lived surface states. 48 Furthermore, the longer component assigned to the reaction of long-lived surface holes with a lifetime of 95 ms is four times Fig. 8 Sub-millisecond to second timescale transient absorption decays at 575 nm of (a) reference, (b) Ta 2 O 5 -overlayer, and (c) Ta-doped hematite photoanodes held at selected bias voltages vs. RHE.…”