Correlating Changes in Electron Lifetime and Mobility on Photocatalytic Activity at Network-Modified TiO₂ Aerogels

Abstract

We use intensity-modulated photovoltage spectroscopy (IMVS) and intensitymodulated photocurrent spectroscopy (IMPS) to characterize carrier dynamics in titania (TiO 2 ) aerogels under photocatalytic conditions. By systematically increasing the weight fraction of the sol-gel precursor during TiO 2 sol-gel synthesis, we are able to impart drastic changes in carrier transport/trapping and improve the photocatalytic activity of TiO 2 aerogels for two mechanistically divergent photochemical reactions-reductive water splitting (H 2 generation) and oxidative degradation of dichloroacetate (DCA). The lifetimes of photogenerated electrons increase in going from lowest-to-highest precursor concentrations, as measured by IMVS, indicating increasing site density for electron traps, a trend that correlates with an 8× improvement for photocatalytic H 2 generation. Electron mobility in the aerogel films, as measured by IMPS, decreases with increasing trap density, further implicating the trapping sites as reactive sites. In contrast, photocatalytic DCA degradation-driven primarily by direct hole transfer to adsorbed DCA-depends only weakly on the electron dynamics in the film. Transient infrared spectroscopy shows no difference in carrier decay amongst the aerogel samples on picosecond timescales, indicating that changes in carrier dynamics within these networked nanomaterials are only observable at timescales measured by IMPV and IMPS. Correlating holemediated and electron-mediated photocatalytic activity with direct measurement of electron dynamics under photocatalytically relevant conditions and timescales comprises a powerful approach to determine how synthetic modifications to networked nanostructured photocatalysts affect the relevant physicochemical phenomena underlying their photocatalytic performance.

Recent Advancements in the Understanding of the Surface Chemistry in TiO2 Photocatalysis

“…Quintero and Nagarajan [55] investigated the adsorption of the organophosphorus molecules dimethyl methylphosphonate (DMMP), sarin, and soman over rutile (110) surface in its clean, hydroxylated and hydrated forms, using a PBE functional. Figure 6 demonstrates the major findings of their study.…”

“…Surfaces 2020, 3 FOR PEER REVIEW 9 respectively) was found to be much larger than for the (101) surface (−184 kJ/mol and −191 kJ/mol for monodentate and bidentate, respectively). Quintero and Nagarajan [55] investigated the adsorption of the organophosphorus molecules dimethyl methylphosphonate (DMMP), sarin, and soman over rutile (110) surface in its clean, hydroxylated and hydrated forms, using a PBE functional. Figure 6 demonstrates the major findings of their study.…”

“…A considerable body of research was performed on Pt/TiO 2 , both under vacuum and in the presence of water vapor [109]. Development of means for measuring transient changes in the IR spectrum at the sub-nanosecond time domain enabled differentiating between trapping at shallow traps (typical time = 15 ps) and trapping at unreactive deeper traps (typical time = 150 ps) in TiO 2 aerogels [110]. Quite counter-intuitively, no correlation was found between the number of electron trapping sites and the decay rate to the deeper traps.…”

$$\hbox {TiO}_{2}$$ TiO 2 aerogel–metal organic framework nanocomposite: a new class of photoanode material for dye-sensitized solar cell applications

“…The photocurrent-voltage (I-V) characteristics and intensity-modulated photocurrent spectroscopy (IMPS)/intensity-modulated photovoltage spectroscopy (IMVS) were investigated using ZAHNER electrik IM6 electrochemical work station coupled with PP211 slave potentiostat to control the LED light source. The electron lifetime (τ n ) was determined from the IMVS spectra and the electron diffusion time (τ d ) was determined from the IMPS spectra by taking the minimum frequency of the semicircle [30]. [31] and introduced on the photoanode.…”

TiO2@Sn3O4 nanorods vertically aligned on carbon fiber papers for enhanced photoelectrochemical performance

“…To further understand the reasons of the charge transfer process occurring at the interface of photoelectrode/electrolyte, electrochemical impedance spectroscopy (EIS) 28,29 was carried out and presented in Fig. 6a and b.…”